OVERHEAD POWFR LINES
ELEMENTARY DESIGN
AND CALCULATIONS
BY
CAPTAIN W. MORECOMBE, R.E.
B.Sc. (ENG.), A.M.I.E.E.
LONDON
CHAPMAN & HALL, LTD.
11 HENRIETTA ST., W.C. 2
1929
3^95
POINTED IN GREAT BRITAIN
I'JY TMK ABERDEEN UNIVERSITY I'RKSS
ABKRDKRN, SCOTLAND
IX
CONTENTS.
Introduction . . . . .
Index to Working Tables x
Index to Working Curves xi
CHAP.
I. Electrical Considerations ....... 1
II. Copper Conductors, Sag and Stress Calculations . . .18
III. Conductor Arrangement, Clearances and Spacing ... 40
IV. Insulators .......... 55
V. Cross Arms and Insulator Brackets 70
VI. Simple Wood Supporting Poles 94=
VII. Compound Wood Poles . 115
VIII. Iron and Ferroconcrete Poles . . . . . .135
IX. Angles and Terminals 163
X. Conductors other than Copper . . . . . .183
XI. Safety Precautions 193
APPENDIX,
I. E.C. Regulations for O.H. Lines 201
II. Information to be furnished to the E.C. when proposing to erect
O.H. Lines 214
III. P.O. Regulations affecting design of O.H. Power Lines in the
neighbourhood of Telegra)h and Telephone Lines H.V. . 219
IV. P.O. Regulations affecting design of O.H. Power Lines in the
neighbourhood of Telegraph and Telephone Lines L.V. . 222
General Index: .......... 233
INDEX TO WORKING TABLES.
PAGE
I. Particulars of Hard Drawn. Copper Conductors ... 4
II. Erection Sags, H.V. Lines, H.D. Copper Conductors . . 24
III. Erection Tensions, H.V. Lines, H.D. Copper Conductors . 25
IV. Erection Sags, L.V. Lines, H.D. Copper Conductors . . 26
V. Erection Tensions, H.V. Lines, H.D. Copper Conductors . 27
VI. Maximum Angles for H.V. Pin Insulators . . . .61
VII. Lengths of Binding Wire required ..... 66
VIII. Particulars of Steel Sections . . . . . .71
IX. Ultimate Stresses, etc., of Timber and Steel ... 73
X. Particulars of Bolts . . . . . . . .75
XI. Basic Loading Values on Standard Span Lengths . . 78
XII. Particxilars of Standard Creosoted Fir Poles ... 97
XIII. Costs of Supports for Various Span Lengths . . . 102
XIV. Suggested Standard Span Lengths for H.V. Lines . . 106
XV. Particulars of Channel Sections . . . . .146
XVI. Particulars of Stay Wires and Stay Rods . . . 170
XVII. Particulars of Various Soils ...... 179
XVIII. Particulars of Various Line Conductor Materials , . .184
XIX. Comparison: Aluminium and Copper Conductors . .186
XX. Comparative Cost : Aluminium and Copper Conductors . 1 87
XXI. Particulars of Galvanised Steel Conductors .... 189
XXII. Sags and Tensions of Galvanised Steel Conductors . . 190
XXIII. Illustrating Economy duo to Steel Conductors . . . 191
XI
INDEX TO WORKING CURVES.
FIG.
6. Voltage Drop per Mile at Various Loads (10 000 volts) .
7. Energy Loss per Mile at Various Loads (10 000 volts) .
8. Percentage Energy Loss per Mile at Various Loads (10 000 volts)
13. Erection Sags, H.V. Lines, 05 sq. in. Copper
14. Erection Tensions, H.V. Lines, 05 sq. fh. Copper
15. Sags of Copper Conductors at 122 F., H.V. Lines
16. Sags of Copper Conductors at 122 F., L.V. Lines .
17. Sags of Copper Conductors at 62 F., H.V. Lines .
"IS. Sags of Copper Conductors at 62 F., L.V. Lines .
25. Measurement of Sags by Swings ......
f)
41. Values of 2 sin ~
49. Strength of Single Poles
51. Chart for Selecting Size of Pole required
55. Buried Depth of Single Wood Poles ....
65. Strength of Rutter Poles
.66. Strength of " A " Poles
67. Strength of " H " Poles
91. Strength of Anchorages ......
:}
:}
PAGE
11
12
13
29
30
. 43
. 60
. 96
to face 101
. 110
132
133
181
ELECTRICAL CONSIDERATIONS 3
If R t , L t) X t and Z t denote the RESISTANCE INDUCTANCE, REAC
TANCE AND IMPEDANCE respectively per 1 000 yards of single con
ductor, then the
a
INDUCTANCE, L t = ( 421 log +  0457 /z) mH.
REACTANCE, X t = 27rfL t .
/, the frequency, is taken as 50 in all calculations in this chapter.
Since the reactance varies with the frequency a correction will have
to be applied to all figures given for voltage drop if the system fre
quency is other than 50
IMPEDANCE, Z t = x/^ 2 + X t \
JO 20 30 40 50 60
Inches Distance between Conductors
FIG. 1. Variation of reactance with spacing.
To simplify matters attention will be confined mainly to the
consideration of eight standard sizes of Hard Drawn Copper Conduc
tors, working particulars of which are given in Table I. Values for
intermediate sizes can be deduced with sufficient accuracy by inter
polation. (The full range of British Standard Solid and Stranded
Hard Drawn Copper Conductors is given in B.S. Spec. No. 1.25.)
The use of other conductor materials is discussed in Chapter X.,
page 183.
The values ofR t in Table I. are taken from the standard specifi
cation, and those of X t have been evaluated from the formula given
above for a spacing of 3 feet and a frequency of 50 cycles per second.
Assuming a constant value for the reactance greatly simplifies
OVERHEAD POWER LINES
i
Overall
Biameter
Wire + Ice,
Weight of
Wire f Ice.
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ELECTRICAL CONSIDERATIONS
the calculations and it will be clear from Fig. 1 that no serious erro
is involved in so doing since in the short span construction con
sidered in this book the spacing will never exceed 5 feet.
A copper conductor of diameter 0 162 inches (0 0201 sq. in.) i
the smallest allowed by the E.G. Regulations for O.TL lines in thi
country, and the stranded conductor 7/ * 193 (02 sq. in.) is the larges
which is, as a rule, justifiable economically. " This will be clear fron
Fig. 2 which shows how the values of R, X and Z vary with th<
crosssection. There is obviously little advantage in using conductor;
larger than 015 to reduce voltage drop, owing to the swamping effec
10
I
I
V
05 JO 15 20 25
Cro?s Section, in Square Inches
PIG. 2. Variation of R, X and Z with, crosssection.
of the reactance. This limitation does not, of course, apply to under
ground cables in which the conductors are much closer together.
Calculation of Voltage Regulation. The vector diagran
in Fig. 3 will be found useful in enabling us to visualise how th<
various factors involved are related to one another.
01 = current vector = vector of reference.
Cos <f) = power factor of load.
OE = transmission voltage.
0V delivery voltage.
OR = VA = resistance drop (RI) parallel to 01.
OX AE = reactance drop (XI) perpendicular to 01.
OZ = VE = impedance drop.
PE = OE 0V = LINE VOLTAGE DROP.
OVERHEAD POWER LINES
It is important not to confuse the impedance drop (VE) with the
true LINE VOLTAGE DROP (PE), which is the arithmetical difference
between OE and OF.
PE
The VOLTAGE REGULATION is defined as the ratio
PE
ov
The PERCENTAGE VOLTAGE REGULATION = ^= x 100 per cent.
Vectors are shown for power factors 08 lagging, unity and 08
leading. 0V = 10 units, and to the same scale RI = 2 and XI =
'B K,W. Constant
O.V. [Constant] '10,000 Volts
C05 M ( R ' e
Lx.i, 
c.,,...(; : "
2,000 v/olts
3,000
^.
2,500
3,750
OE*
, OV 10,000 Volf*
Fia. 3. Transmission line vector diagram.
3 units at unity P.F., the values at 08 P.P. are therefore 25 and 375
respectively. For clearness, these values of RI and XI are much
exaggerated. In practice the impedance drop seldom exceeds 10 %
of the delivery voltage.
From the geometry of the figure we have 0$ 2 = (OF cos d> 4
* + (0V sin $ + XI)*.
If the current leads on the voltage, < must be taken as negative,
ELECTRICAL CONSIDERATIONS 7
in which case, although cos </> remains positive, sin <j> becomes nega
tive.
This expression is rather cumbersome for general use, but the
following simpler formula gives results sufficiently accurate for most
practical purposes with a lagging power factor.
OE = OF + RI cos <f> + XI sin <.
This is deduced as follows : 
Draw EB perpendicular to 0V produced. Then VB is approxi
mately equal to PE.
But VB = VA cos < + AE sin </>
RI cos </> + XI sin <f>.
The results obtained by this simpler formula are exactly correct
for one value of </> only with each size of conductor, viz.,
but over the usual range of lagging power factors the error intro
duced by its use is always inappreciable. For leading power factors,
however, it is necessary to use the more exact formula with sin <f>
negative.
If R and X are values for a single conductor, then the total line
voltage drop will be as follows :
SINGLEPHASE : V.D. = 2(RI cos & + XI sin <f>)
THREEPHASE : V.D. = ^/Z(RI cos </> + XI sin <)
In a simple length of 3phase line without branches,
Watts delivered = W = \/3VI cos $
W
/.I =
. 7 . cos </>"
If this value of I is substituted in the equation for voltage drop
we have
*J~*> W
V.D. = /  v ' (R cos d> + X sin </>)
^ n
OVERHEAD POWER LINES
Assuming F= 100, 7= 10000, /== 50, balanced load and
an equilateral triangle conductor spacing of 3 feet, we get
100000 X l76 /T > , ,
10000
V.D. per mile =
The curves in Fig. 4 have been plotted to illustrate the ADVERSE
40
35
5
9 9 10 : d 8
Leading Lagging
Load Power Factor
FIG. 4. Volts drop per 100 K.W. per mile. 10 000 volts, 3phase, 50 cycles,
3 ft. spacing. Showing effect of power factor.
EFFECT OF Low LAGGING POWER FACTOR ON VOLTAGE REGULATION.
The values for B t and X t being per 1 000 yards of single conductor,
taken from Table I. ; the effect will be seen to be more serious with
the larger conductors.
It will be noted, however, that the introduction of a leading
current at the delivery end of the line by means of condensers or
ELECTRICAL CONSIDERATIONS
9
overexcited synchronous machinery will reduce the line voltage
drop. In certain circumstances it may pay to put in apparatus to
take sufficient leading current to reduce the line drop to zero or even
to produce a rise of volts in the line. This will be seen from Fig. 4
to be quite practicable with the larger conductors.
woo
900
10
9 8 7 '6
Load Power Factor
FIG. 5.
1.0 a o. rower race or
Energy loss per 100 K.W. per mile. 10 000 volts, 3phase, 50 cycle
Showing effect of power factor.
Energy Loss in Conductors. Although the voltage regula
tion will usually be the determining factor in the lines under con
sideration, it is desirable to know approximately the energy loss that
will occur in the conductors. The percentage energy loss is generally
somewhat greater than the percentage voltage drop, the two values
being approximately equal when the load power factor is unity.
10 OVERHEAD POWER LINES
The Total Energy Loss in a 3phase system == 3 . P . R
/ W \ 2
__ r>{ YY \ 7?
= & I T=  T ) it
. V . cos cf>/
2 R
V J * cos 2 <f>'
R being as before the resistance of a single conductor.
The energy loss is therefore inversely proportional to the square
of the power factor.
Taking the same load conditions as those assumed in Fig. 4, we
get :
ENERGY Loss PER MILE
10 000
>< ><  = 176  , watts.
The curves in Fig. 5 have been plotted from this formula to
illustrate the ADVERSE EFFECT OF Low POWER FACTOR on the
efficiency of transmission.
Power Factor Correction. The importance of high power
factor will be realised from the above, the cost of the conductors
depending upon K.V.A. and not K.W. An equitable charge to
power consumers can, therefore, only be made on a K.V.A. basis.
Sxich a tariff will usually influence them to install apparatus for
power factor correction with advantage to all concerned.
Working Curves for V.D. and Energy Loss. A number
of charts will now be given to facilitate the selection of the nearest
standard conductor to satisfy a given set of conditions. It will be
sufficient for practical purposes to design for a load power factor of
08. This should usually give a safe margin, as during the lighting
hours, when the problem of voltage regulation is most acute, the
P.F. should be much higher than this.
A delivery voltage of 10 000 will be assumed, i.e. the current
taken in the calculations will be the largest experienced per K.V.A.
on a standard 11 000 volt system,
Voltage Drop per Mile (Fig. 6). Since the V.D. is pro
portional to the current, and the current (at constant P.F.) is
proportional to the K.V.A., the curve connecting V.D. and K.V.A.
will be a straight line.
ELECTRICAL CONSIDERATIONS
11
If the load is 500 K.WL at 08 P.F.,.
then V.D. per 1 000 yards
= V3~. I . (R t cos $ + Z t sin <)
and V.D. per mile
_ AT 500000 X 1'76
VoX 7=  X ( oA t +
A/3 X 10 000 X '8 V
R t and Z t being values per 1 000 yards, obtained from Table I. E.g.
for 193 conductor, R t = SQ and ^=355 .. V.D. per mile =
110 (8 X 86 + 6 X 355) = 99 volts.
200
780
160
140
120
to
^
X
30
60
40
20
/6
16
72
^

er
.4
/
7
7
7<?0 2^ ^ 4^0 500 600 700
K. \M. Delivered
FIG. G. Voltage drop per mile. Balanced load. 10000 volts, 08 P.F., 3;
50 cycles, 3 ft. spacing.
12
OVERHEAD POWER LINES
This point is plotted in Fig. 6, and a straight line drawn through
it from the origin. Straight lines for the other seven selected con
ductors are drawn similarly.
The Percentage Voltage Drop can be read off the same chart.
300
20 40 60 80  ,100
Delivered
100  200 300 400 500 600 700
K. WA Delivered
Energy loss per mile. Balanced load. 10000 volts, 0'8 P.F.,
3 phase, 50 cycles.
ELECTRICAL CONSIDERATIONS
ENERGY Loss PER MILE (Fig. 7). If K denotes the
livered, then
T ___
x/3 X 10 000 X 8 8^3
,'. Energy loss per mile = 3 . 1 2 . 1'76 . R t
_ 3 . K* . 176 . R t
' " 192
= 0275 .R t .K*i
e.g. for 193 conductor R t == 86 ohm ;
== J^ amps a t 08 P.F.
18
16
14
12
u
c:
*
\<&
100 200 300 400 500 600 700
K.\^f(. Delivered
\ __% Energy loss per mile. Balanced load, 10 000 volts, 0'8 P
3 phase, 50 cycles.
14 OVERHEAD POWER LINES
and if K = 100 K.V.A., energy loss per mile
= 0275 X 86 X 100 2
= 2365 watts
and so on. Proceeding in this way we are able to plot the curves
shown in Fig. 7.
% ENERGY Loss PER MILE (Fig. 8).
The Energy Loss per mile = 0275 R t K z watts.
The Energy Delivered = 1 000 . K . cos <j> watts.
0275 .R+.K*
= 00344. R t .K.%. ,.
This is also the equation to a straight line, therefore the % loss
need only be calculated for one value of the load with each size of
conductor.
Using 193 conductor for 500 K.V.A.
% Energy loss per mile = 00344, 86 . 500 = 148 %.
This point is plotted in Fig. 8 and a straight line drawn through
it from the origin. Curves for the other conductors are drawn in a
similar manner.
% V.D. AND ENERGY Loss AT OTHER VOLTAGES.
<V VD
, /o VJJ.
For a given conductor and load P.F. V.D. oc I.
If the K.V.A. is constant, I oc =.
.. V.D. oc and the % V.D. oc ^.
% ENERGY Loss.
For a given conductor, energy loss oc I 2 .
If the K.V.A. is constant, and also the P.F., / oc =^,
.. / Energy loss oc =r 2 .
We see, therefore, that "both % voltage drop and the % energy
loss are inversely proportional to the square of the working voltage.
At 6 000 volts', the values are ( j = 278 times the value at
10 000 volts and at 20 000 volts, the values are only one quarter as
great.
ELECTRICAL CONSIDERATIONS '17
station is of the order 50 to 100 K.V.A. for distribution to small
consumers, feeding an area of a few hundred yaids' radius only.
Consumers taking 50 K.V.A. or more should be given a H V
supply direct.
The principles already given for deciding the size .of conductor
for H.V. lines apply equally to L.V. lines, but in new work the stan
dard 400/230 volts, 4 wire, 3phase system should always be adopted.
The neutral conductor should be equal in size to the line con
ductors, ^the neutral drop being always appreciable owing to the
impossibility of maintaining a balanced load.
18
CHAPTER II.
CONDUCTORS, SAG AND STEESS CALCULATIONS.
THE true shape of the curve formed by a uniformly loaded wire
strung between two supports is a Catenary, but when the ratio of
span (L) to sag (D) is large (say) greater than 10 to 1 , it is sufficiently
accurate for practical purposes to assume that it is a parabola 3 and
the calculations are thereby much simplified.
The fundamental span formulas are (Kg. 10)
*lr . m
l = L ,ao 2
9 sag at middle of span in feet.
L = length of span in feet.
= length of wire in span in feet.
W = total loading of wire in pounds per foot run,
r = horizontal tension in wire at centre of span in pounds (assumed
uniform throughout the span).
It is unnecessary to distinguish between I and L when calculating
roltage drop and measuring up length of wire required*
The actual tension in the wire is not exactly uniform throughout
ie span, but gradually increases towards the supports where it has
;he maximum value y 2* + (^ . For the ratios of span length
;o sag recommended in this book it will be found that the maximum
CONDUCTORS, SAG AND STRESS CALCULATIONS 19
tension is only a fraction of 1 % greater than tlie horizontal tension,
and. therefore the discrepancy may be safely neglected.
The total loading W, which produces the tension T, comprises :
1. The dead weight of the wire.
2. The weight of any ice, sleet or snow that may cling to the
wire.
3. Wind pressure.
Basic Conditions of Stress. In order to ensure that a
reasonable factor of safety is allowed for in erecting the conductors,
the Electricity Commissioners have laid down the Basic Loading
Conditions to be assumed (see E.G. Regulations, Appendix I.).
For High Voltage Lines (above 650 volts) these are as follows :
Temperature. 22 F.
Ice Loading. f inch radial thickness.
Wind Pressure. 8 Ib. per square foot horizontally at right
angles to the line on whole diameter of ice covered con
ductor.
Factor of Safety. 2, based on the breaking stress given in
B.S. Spec. 125 for hard drawn copper conductors.
For Low Voltage Lines. In order to permit a lower real factor
of safety the hypothetical ice loading is reduced to ^y inch. All
otlier basic loading conditions are the same as for high voltage
lines.
It may be remarked that the above figures for temperature,
ice loading and wind pressure are supposed to represent the worst
weather conditions likely to be experienced simultaneously in Great
Britain, and are legally applicable in this country only.
In tropical countries it will be possible to work with smaller sags
owing to the absence of ice, but it must not be overlooked that the
wind pressures experienced are frequently greater than in this
country.
Minimum Sag under Basic Loading Conditions. The
startingpoint in the sag and stress calculations, is to find the mini
rawm sag which must be allowed to ensure that the safe maximum
stress is not exceeded under the worst loading conditions.
Constants for the eight standard conductors under consideration
are given in Table I., page 4, the notation being as follows (see
Fig. 11):
20 OVERHEAD POWER LINES
d diameter of conductor in inches (B.S. Spec. 125).
t ^ = thickness of ice coating in inches (f inch for H.V., yV inch
for L.V. lines).
d = <? f 2t = diameter of icecovered conductor in inches.
w = weight of conductor in pounds per foot run (B.S. Spec. 125).
ID i = I'25t(d Q + t) = weight of ice in pounds per foot run.
This assumes weight of ice as 573 Ib. per cubic foot ; B.C.
Regulations specify 57 Ib.
ID W Q j Wf  total vertical load in pounds.
Pd 8d 2 .,,.,.
<p =L= =:= d = wind load in pounds per foot run (acting
12 1 2 o
horizontally).
W = */w*' j p z = total load on conductor in pounds per foot ran.
Pro. 11.
Sag and Tension for Erection Purposes. The weather
conditions at the time of erection are very different from those
producing the maximum stress and it is therefore necessary to deter
mine what tension and sag should be allowed when the wire is erected
in still air at various temperatures in order that the maximum stress
shall not exceed the safe limit under the basic loading conditions.
Perhaps it should be explained that considerations of pole sixes and
conductor spacings demand generally that the sag of the conductors
should be as small as possible. The smaller the sag the lower the
F of 8, hence the necessity for a legal limit.
The calculations are somewhat involved since expansion due to
increase of temperature is accompanied by contraction due to elas
ticity.
The simplest form of these calculations will now be given.
Critical Temperature. Starting from 22 F. with ice loading,
as the temperature rises, the wire elongates and the sag increases up
CONDUCTORS, SAG AND STRESS CALCULATIONS 21
to 32 F., when the reduction in loading due to the melting of the
ice results in contraction due to elasticity. As the temperature
continues to rise the sag increases again until at a certain tempera
ture the sag has again the sa.me value that it had originally under the
basic loading conditions at 22 F. The temperature at which this'
occurs is called the " Critical Temperature," the conception being of
great use in solving sag and stress problems in overhead line con
ductors.
Let D m , 8 m and T m denote the sag, stress and tensile load re
spectively under the basic loading conditions and D c , S c and T c the
values at the critical temperature.
wra w ra
Then D = D = = ^
JJJOU J^ J  f m c/m QT\ >
OJm CJc
T * !f. T inrl <? !fi <?
* o TI/ * m> "Jiii fJo ' TIT ''w
Let 6 e denote the critical temperature,
K the coefficient of linear expansion per deg. F. = 9222 X
10(B.S.S. 125),
and ' M the modulus of elasticity in pounds per square inch =
18 X 10 G (B.S.S. 125).
Then the elongation due to temp, rise = I . K , (# 22)
I
and the contraction due to elasticity = (S m S c ) p.
These values are equal ; therefore, equating one to the other we get
i ""()
WJ ~ 166V 1 "" W.
from which 9 e can be calculated .
For example, substituting the known values for 05 square inch
copper from Table I. for II. V. lines we get
29 200 / 2
.. C = 136 + 22= 158 F,
Values of 6 C are given in Table I, page 4.
To Find Sag and Tension to be Allowed on Erection.
Decrease in Length due to fall in temp, from O c to some other temp. 9,
= 1 C K(B  6),
1 being the length of wire in span at the critical temp.
9 OVERHEAD POWER LINES
w L 2
For the unloaded wire in still air, T.D. = ~ which is constant for
o
one particular length of span and size of conductor,
T.D. = T C D C
and S.D. = S C D C
T, D and S being values for temp. 9,
Increase in length due to increased tension
If Z is the length of wire at temp. 6, then
8  ' 2
This must be equal to me difference between the increase in
length due to elastic extension and the decrease in length due to
thermal contraction.
... (D e >  V) = lJSi(B, 6) ?  l.
At tMs stage no appreciable error is introduced by substituting
L for l c , therefore equation may be written :

D \KM
from which D can be calculated for any values of and i. The first
rt fl I 7" ^
step is to find S c which equals Q . /S c is independent of the
o . Jj c a
length of span since D c oc L 2 .
Substituting known values for *05 square inch copper (3/147)
(H.V. loading) from Table I., page 4, we get
CONDUCTORS, SAG AND STRESS CALCULATIONS
The following table can now be prepared :
Span,
8
8Z>t 3
S C D G
Se
feet.
DC.
.Do 2 
s/a 3 '
3A'L 2 "
KM'
KM
100
755
57
2892
lft5
3015
3994
150
170
289
1285
371
678
3994
200
302
91
723
659
1206
3994
250
472
223
463
1030
1884
3994
300
679
460
321
1483
2714
3994
1 Z 3 4
Sag* In Feet
FIG. 12. Graphic method of solving cubic equations. 3/'147 copper conductor,
H.V. loading, 250 ft. span.
Substituting all the known values in equation for D we liave for
a 250 feet span.
158  6 = 103  463D 2 + ^^  3994
1884
D
D
+ 9494,
1884
e.g. if 9 = 62 we have 463D 2 ~~ + 3294 =.
i,e. D 3 = 408 712D and D = 2765 feet.
OVERHEAD POWER LINES
TABLE IT. Hard Drawn Copper Conductors. Sags in Still Ait for
Erection Purposes. High Voltage Lines (fw. Ice).
Span in
Feet.
Temp.
Paht.
162.
193.
3/H7.
8/18.
7/136.
7/166.
7/193.
Sq.
ins.
02061
02926
05
075
10
15
20
feet,
feet.
fpet.
feet.
feet.
feet.
feet.
150
122
225
157
119
106
092
092
092
82
lOO
102
078
071
063
062
063
02
137
083
065
060
053
053
054
42
110
008
055
051
046
046
047
22
087
or>7
048
044
044
040
041
22
306
234
170
133
1035
55.5
765
200
122
462
335
238
199
166
162
158
82
404
204
170
140
118
115
112
02
372
226
142
118
101
098
097
42
340
192
120
101
087
080
085
22
304
161
102
088
077
075
075
22
5U
418
302
236
184
152
136
250
122
700
570
402
329
260
251
245
82
714
490
318
249
197
195
180
02
($83
400
277
214
170
157
157
42
05.1
419
239
183
. 147
139
137
22
022
378
205
159
129
123
121
22
850
653
472
369
2873
238
212
300
122
1140
858
605
486
387
357
341
82
1088
787
515
392
300
275
262
62
1000
750
469
347
262
240
229
42
1032
711
422
304
230
210
202
22
1003
071
374
265
201
185
179
22
1224
9405
679
531
414
342
306
350
122
L _ VIII
850
678
535
484
464
82
758
574
432
392
368
02
,
7 10
523
384
340
325
42'
660
471
340
301
289
22
010
420
298
207
256
22
924
723
364
405
416
400
122
1130
893
705
628
592
82
1037
787
592
515
482
62
988
730
535
462
432
42
938
673
482
413
386
22
888
616
431
370
346
22
'
1208
944
736
608
544
Sags under basic loading conditions shown in italics.
CONDUCTORS, SAG AND STRESS CALCULATIONS 25
TABLE III. Hard Drawn Copper Conductors. Tensions in Still Air
for Erection Purposes. High Voltage Lines (f m. Ice).
Span in
Feet.
Temp.
Faht.
1P2.
193.
3/147.
3/1S.
7/136.
7/160.
7/193.
Sq. ins.
02061
02926
05
075
10
15
20
Ib.
Ib.
Ib.
Ib.
Ib.
Ib.
Ib.
150
122
99
200
473
795
1220
1820
2460
82
134
308
722
1 190
1780
2700
3590
62
163
380
865
1405
2 120
3150
4190
42
203
462
1020
1655
2440
3 630
4800
22
256
550
1 170
1920
2740
4180
5510
22
633
874
1 457
2 125
2935
4265
5635
200
122
86
168
420
755
1200
1840
2540
82
99
214
589
1070
1700
2580
3590
62
107
249
704
1270
1980
3040
4140
42
117
294
833
1485
2290
3460
4730
22
131
350
980
1705
2600
3960
5360
22
633
874
1457
212$
2035
4265
5635
250
122
81
155
389
715
1 170
1850
2570
82
87
177
491
945
1580
2380
3490
62
91
191
564
] 095
1840
2950
4000
42
95
210
655
1280
2120
3340
4580
22
100
233
761
1475
2620
3770
5 180
22
633
874
1457
2 125
2935
4265
5635
300
122
78
148
372
695
1 160
1870
2650
82
82
161
437
860
1490
2430
3 450
62
84
169
480
975
1710
2790
3950
42
86
178
533
1 110
1950
3190
4470
22
89
189
600
1275
2230
3610
5050
22
633
874
1457
2125
2935
4265
5635
350
122
360
680
1 140
1880
2650
82
403
800
1415
2320
3340
62
431
880
1 590
2 680
3780
42
463
975
1800
3020
4250
22
500
1090
2 050
3410
4 800
22
1457
2 125
2 935
4265
5635
400
122
355
670
1 135
1 890
2720
82
.
386
760
1 350
2310
3 340
62
.
406
820
1490
2 580
3720
42
428
890
1 660
2 880
4160
22
f,
451 ,
975
1 850
3220
4 650
22
, ,
1457
2 325
2935
4265
5 635
Maximum working tensions allowed by E.G. Regulations shown in italics.
OVERHEAD POWER LINES
This may be solved by slide rule or graphically as indicated in
Fig. 12 In either case a table of cubes of numbers will be found
useful.
Having obtained the sag in this way, the tension can. easily be
found since TD is constant and equal to
we have
2 X 25Q 2
2 765 X 8
8
= 565 Ibs.
For 3/147 at 62 F.
TABLE IV. Hard Drawn Copper Conductors. Sags in Still Air
for Erection Purposes. Low Voltage Lines (f\in. Ice).
Span in
Peek
Temp.
Faht.
136.
162.
193.
3/147.
3/18.
7/136.
7/166.
7/193.
Set. ins.
01453
02061
02926
*05
075
10
15
20
feet.
feet.
feet.
fpet.
feet.
feet.
feet.
feet.
100
122
044
041
039
041
043
039
041
042
82
028
026
026
027
028
026
027
027
62
024
022
022
023
023
022
023
023
42
021
019
019
020
020
019
020
020
22
018
017
017
018
018
017
01S
018
22
100
077
0615
048
0395
032
028
020
150
122
137
.107
097
094
095
086
089
0*90
82
088
070
065
064
064 '
059
060
062
62
072
058
055
054
054
051
052
053
42
060
050
017
047
047
044
045
046
22
051
044
041
042
042
039
040
041
22
225
173
1385
108
089
072
063
0585
200
122
300
223
188
171
166
149
150
154
82
228
157
131
121
M7
106
107
110
62
193
132
Ml
103
100
092
092
095
42
161
Ml
095
089
087
080
081
083
22
134
095
082
078
077
071
071
073
22
400
308
246
192
158
128
112
104
250
122
521
387
314
273
258
228
227
231
82
442
303
234
202
190
169
167
171
62
403
262
200
174
164
146
146
148
42
361
224
372
161
143
129
128
130
22
318
192
148
132
126
114
314
116
22
625
481
384
300
247
200
175
1625
300
122
779
595
477
408
369
323
318
319
82
704
502
382
318
283
246
242
245
62
6ttt
456
336
278
247
216
212
215
42
620
408
293
243
216
190
186
189
22
575
361
255
213
191
170
167
169
22
000
693
554
432
356
288
252
231
Sags under basic loading conditions shown in italics.
CONDUCTORS, SAG AND STEESS CALCULATIONS 27
It will be realised from the above example that the calculation
of sag and tension is no light task to be undertaken on the spur of
the moment. Hence the necessity for working tables. Tables II.
and III. give values of sag and tension for High Voltage lines (f in.
ice) and Tables IV. and V. for Low Voltage lines (iVin. ice).
Charts for erection purposes can be prepared from these tables.
Curves for 3/147 (05 sq. in.) are plotted in Figs. 13 and 14.
TABLE V. Hard Drawn Copper Conductors. Tensions in Still Air
for Erection Purposes. Low Voltage Lines (f^in. Ice).
Span
in
Feet.
T<nnp.
JFaht.
336.
162.
193.
3/147.
3/18.
7/136.
7/166.
7/193.
Sq. ins.
OH53
02 061
02926
05
075
10
15
20
Ib.
Ib.
Ib.
Ib.
Ib.
Ib.
Ib.
Ib.
100
122
160
245
360
610"
870
1280
1810
2390
82
250
385
540
925
1 350
1920
2800
3600
62
290
450
640
1090
1600
2220
3230
4290
12
335
525
740
1250
1850
2600
3720
4960
22
390
600
840
1390
2060
2900
4220
5580
22
456
633
874
1457
2125
2935
4265
5635
150
122
115
208
327
600
890
1300
1890
2510
82
179
318
488
880
1320
1900
2790
3630
62
219
385
578
1040
1550
2200
3210
4260
42
263
445
675
1200
1790
2550
3700
4900
. 22
309
507
775
1340
2010
2870
4180
5500
22
456
633
874
1457
2125
2935
4265
5635
200
122
93
178
300
585
900
1340
1980
2610
82
123
253
430
825
1280
1880
2780
3660
62
145
300
508
970
1500
2170
3200
4240
42
174
358
593
1 125
1720
2490
3670
4850
22
209
418
688
1285
1950
2810
4130
5460
22
456
633
874
1457
2125
2935
4265
'5635
250
122
84
160
28a
570
910
1370
2040
2720
82
99
205
376
775
1230
1850
2770
3670
62
108
237
440
900
1430
2130
3170
4220
42
121
277
511
1 035
1640
2420
3 640
4820
22
137
323
595
I 185
1870
2740
4060
5400
22
456
633
874
1457
2125
2935
4205
5635
300
122
81
150
266
550
915
1390
2100
2830
82
90
178
332
705
1 190
1 820
2760
3690
62
95
196
378
810
1 370
2070
3150
4200
42
102
219
432
925
1 560
2 350
3 610
4780
22
no
248
497
1 055
1 760
2 040
4 000
5 350
22
456
633
874
1467
2125
2935
4265
5635
Maximum working tensions allowed by E.G. Regulations shown in italics.
28 OVERHEAD POWER LINES
It will be noted that on the shorter spans the variation of tension
with temperature is considerable. The undesirability of using spans
of unequal length will be at once clear. If contiguous spans differ
very much in length, changes of temperature will cause longitudinal
pulls which may result in the conductors slipping at the binders.
Variations in span length should not exceed 10 % of the average.
The binders will stand up to this difference and the poles are flexible.
In cases where abnormally short or long spans are unavoidable,
tensioning insulators and longitudinal stays must be used. The
remarks in this paragraph do not apply to lines using suspension
insulators.
Sag at 122 F. in Still Air (see Figs. 15 and 16). These
curves (plotted from Tables II. and IV.) are required for determin
ing the height of pole required. The Fig. 15 curves also appear in
the first quadrant of Chart (Fig. 51, page 101).
It may be worth noting that for conductors up to ! sq. in.' on
H.Y. lines and up to 05 sq. in. on L.V. lines the oblique sag under
basic loading conditions is greater than the vertical sag in still air
(at 122 F.
Sag at 62 F. in Still Air (see Figs. 17 and 18). These curves
(also plotted from Tables II. and IV.) are required when determining
the horizontal spacing between conductors, which will shortly be
considered.
Sag and Tension under various other Loading Con
ditions. Although the sag and tension tables given are sufficient
for practical purposes, the following calculations will be found
interesting and instructive.
Vertical Sag at 22 F. with f inch Ice only (no wind).
Let D m) S m and W^ *efer to basic loading conditions and D i: Si and
W i to ice loading only ;
W L z W T2
then D m = 4^~ and 7), = ^,
W m T f WJS<
Whence & = S m .
D<W m '
The Elastic Contraction due to removal of wind
7 Q
l ud
U voltage lines ! l 5
O Mr,
B'lore
30
OVERHEAD POWER LINES
400
350
300
250
200
150
100
3456
Sag, in Feet
300
FIGS. 15, 16. Sags of copper conductors at 122 F.
IDUCTORS, SAG AND STRESS CALCULATIONS 31
2345
Sag, in Feet
of copper conductors at 62 P.
32 OVERHEAD POWER LINES
This is equal to the change of length of wire in span
Equating one to the other, and putting // for I, we get
' ra
87lf V D,W
i '' in
Substituting known values for 05 sq. in. copper conductor and. 251
feet span we get
4 7 p 3 _ D a 3 X 29 200 X 250 2 / 472 X 521^
8x18x10 \ A~X '88 /'
Whence JD, 3 + 157D, 1063 =
and J3 4 = 366 feet.
2  521x25Q2
NOW
8D t 8x 366 "
HP = 22 200 Ib. / sq. in.
Vertical Sag at 22 F. in Still Air Without Ice. As
above, if D , j$ and W refer to these conditions we have
ns ni_ 3 WVi AJF
.^02 n 2 3 X 29 200 x 250V, 472 X 2
i.e. aM  /y = ______ M
a x la x 10 \ D Q x i
Whence D S  157D ? 408 == and D = 205 feet, which agrees
with, the value given in Table II. which was prepared from equation
on. page 22.
Vertical Sag at 32 F. with finch Ice only (no wind).
In this case we have
THERMAL EXPANSION = Z/C(32 22) = IQIK.
Starting with the iceloaded conductor at 22 P. in still air, w is
constant, therefore
Substituting for 8 32 and putting L for I we get
'
_
Si JK D
'aa
CONDUCTORS, SAG AND STRESS CALCULATIONS 33
Inserting known values
n 2 O/VM, 30 X 9222 X 10~ 6 X250 2
3 X 22 200 x 250V _ 366
8X18 X 10~ G ( ~ D
__' 13.4 = 216  289
"Whence 7) 32 3 + 1331Z> 32  1056 =
and D 32 ^=38 feet.
Oblique Sag at 32 F. with finch Ice and 8 Ib. wind.
Starting from basic loading conditions
THERMAL EXPANSION = Z#(32 22) = lOZ/i
ELASTIC CONTRACTION = (S m S 32 ) ~
W is constant .. S m l) m S.^D' SZ .
Substituting for $ 32 and putting L for I we have
Inserting known values
30 X 9222 x 10 X 250 2
3 X 29 200 X 250V 472
8X 18 X 10
1795
D 3 o 2  223 = 216  38 + ^r
L'sa
Whence D 32 3 + 1354D 3a  1795 =
and D 32 = 485 feet.
250 V. 472\
^~( L ~irJ'
The maximum hypothetical horizontal displacement of the con
ductor (485 x = 392 feet) occurs with this loading.
'88
Oblique Sag at 62 F, with 15 Ib. Wind. The wind
pressure of 8 Ib. per square foot corresponds with a wind velocity
of 50 miles per hour, which it is considered sufficient to allow simul
taneously with an ice loading. It is not the highest value likely to
be experienced ; in fact, velocities exceeding 100 miles per hour have
been recorded and 70 miles per hour is quite common at higher
3
34 OVERHEAD POWER LINES
temperatures. This is of no importance so far as the strength of
the wire is concerned but it may cause trouble due to the wires
swinging together (see p. 42). We will therefore consider the state
of affairs with 15 Ib. wind pressure at 62 F.
Let the subscript " w " refer to values for the wind loaded wire
and " o " to the wire in still air.
,
QJ. o
from which T w = T Q . ^ and ^ =
"" QLsw
Increase of Stress due to wind loading
W T)
"'
From Table I., weight of conductor per foot run = 2 Ib. and diameter
= 317 inch:
IK
Wind pressure = 317 X ^ = 396 Ib. / ft. run
and W u = V'396 2 + 2 2 = 443 Ib. / ft. run.
From Tables II. and III., D = 2 765 feet, and T = 505 Ib.
565 *
...S = s _= 11 300 Ib. / sq. inch.
Now, substituting known values in equation
we get
D 2 _ 9. 7652 _ 3 X 11 300 X 250V 443 X 2 765
w " ~~
Ijghence D^s + 712^ _ 90 =
and , /), = 395 feet. ..
The triangle of forces acting on the wire is shown in Fig. 19.
Therefore the horizontal displacement of the wire will be
.OQtf
3 ' 95x  = 354 feet.
J96
CONDUCTORS, SAG AND STRESS CALCULATIONS 35
The above results are shown diagrammatically in Fig. 20. The
angles of deflection for 7/122 aluminium and 7/132 steelcored
aluminium, which are electrically equivalent to 3/147 copper, are
shown also for comparison.
The figures shown are for a span length of 190 feet in the case of
7/122 aluminium and of 310 feet in the case
of 7/132 steelcored aluminium, these span
lengths requiring about the same sag at
122 F. in still air as 3/147 copper on a 250
feet span.
(See Chap. X., p. 183, and Tables XVIII. FlG ' 19<
and XIX.)
This figure will be found useful when considering the horizontal
spacing to be allowed between conductors (see p. 42).
368'
15 IBs. Wind
62"F.15lbs.Wind
15lba.Wind
^a Ice, 8lbs. Wind
62 F Still Air (
22 F 3 4lce, 8 Ibs, Wind N % X 22F. 9& Ice. fl Ibs.Wind
 !32"F. %' Ice, 8 Ibs. Wind
392' >J
22 F.9& Ice, Still Air
32"F.%"lce,StiUAir
12ZF. Still Air ^ 4 . 02  @4 . 15 .
0) Copper 3/ 147 250 ft. Span
Aluminium 7/122 190
Steel Cored Aluminium 7/132 310
FIG. 20. Diagram to illustrate movement of conductors under various loading
conditions. (H.V. loading.)
Notes on the above Calculations. No great precision ota
be claimed for the figures given in the Tables II. to V.
We cannot be certain of the exact values of the modulus of elas
ticity and of the coefficient of linear expansion, and tolerances of
1 % and 2 % are allowed on the conductor diameter and
weight respectively.
But it must be realised that the lineman cannot in practice avoid
36 OVERHEAD POWER LINES
errors of a few inches or so when adjusting the sag and the tempera
ture at the time of erection can only be estimated. Even if a ther
mometer is used, the temperature of the wire in a hot sun may be
much greater than the air temperature.
The Tables show the minimum sag required by the Regulations,
but it is not always necessary or desirable to pull up to the legal
limit. There is no point in pulling up to sags less than (say) 1 foot.
Frequently in short span work, larger sags are given than are neces
sary from a legal point of view in order to reduce the value of the
terminal stresses.
Unfortunately, the percentage increase of sag to be allowed on
erection in such cases is much greater than the desired percentage
decrease in terminal stress under the worst loading conditions. As
the Tables cannot therefore be used, the following method of cal
culating sags and. stresses is suggested.
Erection Sags and Tensions with Reduced Basic
Loading Stress. Having decided upon the maximum stress
to be allowed, first find the sag at the critical temperature and then
the sag at 22 F. without ice and wind.
The sag (and stress) at other temperatures may then be found
with sufficient accuracy for practical purposes by assuming a straight
lino law of variation of sag (and stress) with temperature.
For example, consider a 7/166 conductor on a low voltage line
pulled up to half the maximum tension allowed by the Regulations,
i.e. 14 250 Ib. / sq. in. under basic loading conditions (Table I., p. 4).
(1) Sag at Critical Temperature.
14250/ 595\
""^""" ~'
Whence B = 54 F.
Assume a 200 feet span.
From Table III., page 25, the sag under basic loading condi
tions ( T \in. ice) with 'S m = 28 500 Ib. / sq. in. = 112 feel There
fore the sag under the same loading conditions with half the stress
will be increased to l]2 X 2 = 224 feet and this will also be the
value of the sag at the critical temperature 54 F.
(2) Sag at 22 F. without Ice and Wind (see p. 32).
r> 2 _ 7) ?, ^rnL'
m 8M
CONDUCTORS, SAG AND STKESS CALCULATIONS 37
Inserting known values, we have .
12 n 2 __ 3 X 14 250 X 200V, 224 X 595\
"
Whence
We now have
and
8 X 18 X 10 6 V 1
D 3 f 688 DO  1665 = 0.
D = 170 feet.
5=22F., D = 170 feet,
B '= 58 F., D = 224. feet.
X 95 /'
35
30
25
True Sag for
S m m. 14,250 lbs./sq.in,
under Basic Load
Ing Conditions
20
CO
75
10
05
Straight Line
through Points
Sag
20 40 60 80 100 120 140
Degrees Fahr.
Fro. 21. Erection sags for 7/100 copper, 200 ft. span. Low voltage linos.
These points are plotted in Fig. 21 and the straight line drawn
through them is seen to be in close agreement with the more exact
curve obtained in the manner described on pages 2123. As a
matter of fact, this shorter method of treatment gives fairly good
results in most cases, but it is not advisable to use it when
working at stresses near the legal maximum.
Solid Versus Stranded Conductors. For sections above
01 sq. in., stranded conductors must be used as solid conductors
38 OVERHEAD POWER LINES
become too unwieldy to handle, but between about 03 and 075
sq. in. practice differs according to the experience of the engineer;
Stranded conductors are undoubtedly easier to handle arid less liable
to serious damage due to want of skill or carelessness.
For the same crosssection, the stranded conductor is a little
more expensive than the solid ; the stranded has the larger diameter
and therefore a larger wind load, but on the other hand the safe
working load of the stranded conductor is higher, and it will be
found that the sag to be given to a conductor of given crosssection
is sensibly the same whether the conductor is stranded or not. It
may be noted that it is not usual to carry the stranding so far with
H.D. copper conductors as with the annealed copper conductors
used in cables. For example, a standard 15 sq. in. conductor has
7 strands of 166 inch diameter in overhead line work and 37
strands of 072 inch diameter in cable work.
Notes on Sag Adjustment During Erection. It will be
found that when the conductor is erected, the final tension to be
allowed is relatively low and may be insufficient to smooth out any
slight kinks there may be and to take the initial stretch out of the
material.
It is usual, therefore, to pull up the conductor to GO % of its
breaking load (i.e. about 20 % greater than the safe working load
values given in Column 8, Table I., p. 4) and to maintain this load
for a few minutes. It may then be assumed that for practical pur
poses, the stress will be proportional to the strain over the normal
working range which is, of course, assumed throughout in the
above calculations. This applies to both solid and stranded con
ductors.
If it is impracticable to apply such largo loads as the above implies
with the larger conductors, then as great a load as possible should be
applied and maintained for a couple of clays or so.
Having " killed " the wire in this manner, the final sag adjustment
can be made as follows :
(1) Points of Support at Same Level (Fig. 22 (a)).
Mark oil on the supports the distances AM and BN each equal
to the appropriate sag from Tables II. or IV. and then .adjust the
conductor until it just appears in the line of sight between M and N.
If the sag is small it is desirable to check the result with a spring
dynamometer, using the figures in Tables III. and V. ' Another
method of checking the sag is referred to on page 42.
CONDUCTORS, SAG AND STRESS CALCULATIONS 39
(2) Points of Support at Different Levels (Fig. 22 (6)).
The treatment in this chapter is only strictly correct when the
supports are on the same level. When the spans are long and the
differences of level large, the design requires special consideration,
which is outside the scope of this book, but for short span con
struction on moderate slopes no difficulties are likely to arise from
using the tables of sags and stresses given and proceeding as above.
The length of span must be taken as the horizontal distance
between the supports (L) in all cases. On a slope of 1 in 3, however,
Fro. 22().
FlG. 22 (b).
the length of slope I/(Mg. 22 (6)) exceeds L* by about 6 % only. It
is to be noted also that if the ground surface is parallel to AB and
MN, then the vertical distances MP and QH are about 6 % greater
than the minimum ground clearance GJ. Therefore, for a given
horizontal distance between the supports the poles will have to be
somewhat longer on an incline than on level ground.
There may be an upward pull at the lower support A, but in
ordinary circumstances this is of no importance when pin insulators
are used.
CHAPTER III.
CONDUCTOR ARRANGEMENT, CLEARANCES AND
SPACING.
We have to consider
(1) Clearance of line conductors from ground.
(2) Clearance of earth, wire and auxiliary conductors from ground.
(3) Clearance of line conductors from pole and pole ironwork.
(4) Spacing between line conductors. The earth wire may be
considered as a line conductor as far as spacing is con
cerned.
The clearances for (1) and (2) are fixed by the E.C. Regulations
(see Appendix I.) and are as follows. The figures are the minimum
allowed by the regulations at 122 K, the assumed maximum
slimmer sun temperature in this country :
(1) Clearance of Line Conductors from Ground.
(<?.) HIGH VOLTAGE LINES.
In all situations at all voltages up to G6 000 20 feet.
(b) Low AND MEDIUM VOLTAGE LINES.
(i) Public road crossings . . . 19
(ii) Situations inaccessible to vehicular traffic 15
(iii) All other positions . . . 17
(2) Clearance of Earth Wire and Auxiliary Con
ductors from Ground.
(a) HIGH VOLTAGE LINES.
(i) When erected across a public road or canal
or across a railway . . . .20 feet,
(ii) Situations inaccessible to vehicular traffic 15 ,,
(iii) All other positions . . . . 17
(b) Low AND MEDIUM VOLTAGE LINES.
(i) Public road crossings . . . 19
(ii) Situations inaccessible to vehicular traffic 15
(iii) All other positions . . . 17
CONDUCTOR ARRANGEMENT
41
(3) Clearance of Conductor from Pole and Pole Iron
work. It will be noted in Kg. 38, page 56, that the dry spark
over distance on a typical 11 000 volt insulator is about*? inches.
This may be taken as a rough guide to the working clearance which
should be allowed between conductors and metal crossarms,. earthing
brackets, etc. The following minimum values are suggested :
Up to 660 volts
6600
11 000
22 000
33 000
4 inches.
6
9
10
12
Bird Trouble. A good deal of trouble is sometimes experienced
due to birds settling on the wires or pole ironwork and causing short
circuits or earths, mainly the latter. The result is generally disas
OR PORCELAIN.
FIG. 23.
Fia. 24.
trous for the bird but unfortunately its electrocution frequently
causes an interruption of supply due to the operation of the leakage
protective device. This, of course, only occurs on high voltage
systems, and the higher the voltage the greater the nuisance.
To obviate the trouble we may either,
(a) ALLOW LARGER CLEARANCES, as for example by using
longer insulator pins than would otherwise be necessary.
(b) DISCOURAGE BIRDS PROM SETTLING.
The designs shown in Figs. 30, 31 and 32, pages 4648, appear
to be quite satisfactory. Birds are seldom found to settle on the
slanting surfaces provided.
(c) PROVIDE INSULATED PERCHES OR " BIRD GUARDS."
Two types are illustrated in Figs. 23 and 24. In this connection
it is to be noted that bird guards are not necessary with oak arms, if
the earth connections to the insulator pins are fixed under the arms.
42 OVERHEAD POWER LINES
A rubberwax compound known as Pernax, which is a tough,
flexible and durable insulating material of high dielectric strength,
obtainable in sheet and tube form from the Groydon Cable Works,
can also be recommended for wrapping round arms, conductors
and stay wires.
(4) Spacing Between Conductors. It will be obvious that
the closer the conductors are together the better from a mechanical
point of view, since shorter poles and smaller and lighter pole fittings
can be used.
Up to (say) 33 000 the working voltage has little bearing on the
matter.
The HORIZONTAL SPACING will first be considered. This is
decided mainly from the possibility of the conductors blowing to
gether in strong winds. Smaller conductors should, therefore, have
relatively larger horizontal spacing, not only because the sags are
larger, but also because of the greater ratio of wind loading to weight,
which results in a greater displacement from the vertical for a given
wind pressure. Table XIV., page 106, illustrates this point.
If a \vire hanging freely in a parabolic curve is displaced from the
vertical and then released, it will swing regularly, and, considered as
a compound pendulum, it can be shown that the relation between
the sag in feet (D) and the number of half swings per minute (N} is
given by the equation D 7^2 This relationship is plotted for
sags up to 1 feet, in Fig. 25, which will be found useful when checking
sags during erection.
If, therefore, all the conductors are erected with precisely the
same sag they should swing together synchronously, and there should
be nothing to fear from contacts, but unfortunately exact equality of
sag is difficult to effect and maintain in practice, and consequently
the various conductors may have different periods of swing. Actually,
of course, in a gusty wind the movements are very erratic, particu
larly of the smaller and lighter conductors. From a theoretical point
of view, to render it physically impossible for two conductors in the
same horizontal plane to touch one another, it would be necessary
to allow a spacing equal to twice the maximum horizontal displace
ment of each conductor due to the highest wind pressure likely to be,
experienced. This assumes the conductors to swing 180 degrees out
of phase, but a little thought will show that such a contingency is
very remote, and it is found practically that a spacing about equal
CONDUCTOR ARRANGEMENT
to the maximum horizontal displacement gives a good facto:
safety .
Reference to Table XIV. (p. 106) and Fig. 20 (p. 35) will si
that there is some justification for the following practical rules
H.V. lines. (Somewhat smaller spacings, say, 20 % less, will usu
suffice for L.V. lines, with a minimum of 1 foot) :
HORIZONTAL SPACING (that is, the distance between conduc"
when fixed in the same horizontal plane, as in Figs. 26 and 27) :
Copper. Allow a spacing equal to sag in still air at 62 F.
Aluminium, ,, ,, 1 times ,, ,, ,,
Steel Cored Aluminium. Allow a spacing equal to 1 times
in still air at 62 F.
N? of Half Swings pen Minute (N)
_. ^9 fc. O Co O N
"2. Q Q g c
\
\
\
D= 14.6C
^
^^
~~^^
Z 4 6 8 10 12
Sag, In Feet (D)
Fio. 25. Measurement of sags by swings.
VERTICAL SPACING. In this case there is no danger of the TA
swinging together if equally loaded, but we have to consider
possibility of an upper conductor becoming more heavily IOE
than a lower one due to unequal quantities of ice or flocks of b'
Moreover, a numbe'r*of birds settling on a lower conductor at s
distance from centre of span may cause the conductor to be li
up in the other half of the span sufficient to make contact with
upper conductor. It is generally inadvisable to fix high vol
44 OVERHEAD POWER LINES
conductors above one another exactly in the same vertical plane for
these reasons (Fig. 28).
Logically, the vertical spacing should bear some relationship to
the ratio of ice load to the weight of the wire alone, but the practical
rule which is usually worked to is :
VEETICAL SPACING (Copper Conductors). Allow 1 foot per 100
feet length of span (with a minimum of 1 foot).
This spacing gives a reasonable factor of safety for conductors
up to 3/18 (075 sq. in.), but is on the generous side for the
larger copper conductors.
Perhaps it should be made clear that by " vertical spacing " is
meant the vertical distance between horizontal planes through the
conductors. on the same side of the pole.
Somewhat larger vertical spacings should be allowed with the
smaller aluminium and steelcored aluminium conductors.
It is to be remarked, however, that a large number of lines, both
in this country and abroad, appear to be giving satisfactory service
with spacings much less than these rules demand. On, long spans,
of the order of 600 to 900 feet, it is observed that the wires do swing
synchronously, the deflection in strong winds being several times as
great as the spacing between conductors. It will be noted in Fig.
25 that the rate of change of N with D falls rapidly with the larger
sags, and, further, the small differences in sag inevitable in erection
are comparatively insignificant, expressed as percentages. More
over, comparatively large conductors are generally used on these
long spans.
Arrangement of Conductors.
HIGH VOLTAGE LINES. Figs. 26 to 32 show examples of con
ductor arrangement on single circuit H.V. distribution lines, with
earth wire but without auxiliary conductors for protective gear or
telephones. The clearances shown in the figures are suitable for
3/147 conductor on 250 feet spans. The mechanical strength of the
pole fittings will be considered later.
It will be found generally that a horizontal arrangement of con
ductors allows a shorter polo to be used and this may be of importance
in some cases, unless the vertical arrangement is indicated by other
considerations.
An equilateral triangular arrangement (Figs. 20, 27 and 32) is
best from a purely electrical point of view and is therefore desirable,
if nothing is lost thereby ; but this is not a ruling factor in
CONDUCTOR ARRANGEMENT
'"! 9 j 2 f.
Scale.
Rivcftcd.
Chonnal.4xZ'
9"
.1
 _
29"
"^cS 1
Fio. 20.
rfht. Wire.
/
ChoBnel.SI'/i
i___
. 2'9'
I
26"
Fia. 27.
Earfh Wire.
FIG. 28. Fio. 29.
Simple typos of H.V. polo fittings.
OVERHEAD POWER LINES
FIG. 30. Type of H.V. pole fittings.
CONDUCTOR AURA NGEMENT
channel
Scale "to Eastern fe/y
N3/V C/famnce f/o/fs
FIG. 31, Type of H.V. pole fittings.
48
OVERHEAD POWER LINES
T , i , I ,___?,,.
onn , 's standard design.)
20 000 volts, double insulators and wire guards.
CONDUCTOR ARRANGEMENT 49
distribution work, and if the nearest standard pole is a little on the
short side, the vertical distance between the conductors in these
three designs may safely be reduced to 30 inches.
Figs. 30 and 31 are examples of the " tilted " triangular arrange
ment which is favoured by some engineers, there being no two
conductors in the same plane, horizontally or vertically. It can
easily be verified by trial that, for the same spacing between con
ductors,' the factor of safety against contacts is greater than with the
arrangement of Figs. 26, 27 and 32, in which the two lower conductors
axe in the same horizontal plane.
i\ Figs. 26 and 32 show double insulators which are specified by the
\\ Electricity Commissioners in the neighbourhood of roads.
Although the conductor clearance from the ground must be at
least 20 feet in all cases, a clearance of 15 feet only is required for the
earth wire across country, and it may, therefore, be two or three
feet lower than shown in the figures. Advantage may be taken of
this to ease up the load on the pole, but the higher the earth wire is
fixed, the more effective it is as regards atmospheric effects.
Low VOLTAGE LINES. Some examples of L.V. conductor
arrangement are given in Figs. 33 to 36.
L.V. distribution is invariably short span work on account of
service connection which must be taken of! at the poles, and for
which reason it is frequently more convenient to arrange the con
ductors in a vertical plane (Figs. 35 and 36) in spite of the somewhat
longer poles thereby necessitated.' The use of poles for street light
ing also has a bearing on the question of span length. Owing to the
relatively short spans and the less onerous hypothetical loading
conditions it is not usually necessary or desirable to pull up the
conductors to the limit of tension allowed by the regulations, as the
increased difficulty and cost of dealing with the stresses at angles and
terminals is far greater than any saving which might be effected in
the cost of supporting poles.
It is, of course, necessary, mainly for aesthetic reasons, to allow
the same sags on all sizes of conductors on the same poles, but in
cases where this point arises, the span length seldom exceeds 150
feet.
Fig. 3.3 shows an arrangement suitable for a 4wire L.V. feeder
(without branches), with a " split " neutral, which was required by
the old (1923) B.C. Regulations. The new (1928) Regulations permit
a single wire neutral to l?e used with this design, provided the wire
4
OVKKIIKA1) POWER LINKS
Eta. 34.
Typos of L.V, polo fittingB.
CONDUCTOR ARRANGEMENT
. 30.
Typos of L.V. polo fittings.
52. OVERHEAD POWER LINES
is staggered from one side of tlie pole to the other, but this appears
to present practical difficulties.
Pig. 34 shows 3phase wires, 1 switch wire, and a " split " neutral,
also erected to comply with the old Regulations.
Fig. 35 shows a vertical arrangement which now complies with
the Regulations, the single wire neutral being considered sufficient
as a guard wire when it is directly below the phase conductors.
Kg. 36 shows a vertical arrangement with a " V " guard, re
quired by the old Regulations, but now no longer necessary.
As the insulation of the shackle (Kg. 35) is inferior to that of the
shed insulator (Fig. 36) it is better to use the latter type as far as
possible in straight rims and for small angles, and reserve the former
for terminals and considerable angles.
H.V. and L.V. Lines on Same Poles. The idea of using
the same poles for both H.V. and L.V. lines is, of course, by no means
new. It has been common practice on the Continent and in America
for many years, but has hitherto not been encouraged in this country.
Fig. 37 shows a combined pole which has recently been approved
by the Electricity Commissioners and which presents many points
of interest.
The Shropshire, Staffordshire and Worcestershire Electric Power
Company have gone very carefully into the question of standardisa
tion and the design shown is typical of their standard practice in
rural distribution. All drilling and slotting of poles is done before
despatch to site. Every pole is slotted to take an 8 in. X 4 in. X 4
ft. foundation block 1 foot 6 inches below ground. This enables a
foundation block to be readily fitted if, on excavation, the engineer
decides that the nature of the ground is such as to make it necessary.
The buried depth of 5 feet G inches should be ample for most common
types of soil, but the 1928 E.G. Regulations are really more
onerous than the old ones concerning pole foundations.
The vertical spacings between conductors is rather less than is
recommended elsewhere in this book, but the values allowed have
been found to give satisfactory service in the area of supply con
cerned.
The minimum clearance between H.V. and L.V. lines is 19 inches.
The pole brackets will be seen, to be of very simple design. Ordinary
standard insulator pins are used with distance pieces of iron tube
between the strap brackets. One L.V. phase conductor only is shown,
but the poles are of sufficient diameter and length to accommodate
CONDUCTOR ARRANGEMENT
3phase conductors, when required. The L.V. phase conductors
are insulated with P.B.J. insulation (see Specification, Appendix
IV., p. 231).
The short pieces of straight iron strap marked " a " on the L.V.
Carfbcd
Ctonhnl Conductor.
05'= fcD97 H.D.
porcclairj
lasulaiors
Drowc
porcukiit2 Irasalatbrs.
Ebrl'hiGg Bow
Hxterads OutwardsH'.
05"" 7097.
R5.J. Insulated conductor.
wre
bare copper.
LEMGTH or POLES. 54 1 . Dia: AT 3' FROM
MAXIMUM SPAN. 150'.
MINIMUM CLEARANCE BETWEEN H.V. <&LV. LINES. H9".
i DEPTH In GROUPID. 5'6".
PIG. 37. H.V. [3 300 volts] and L.V. [230 volts to earth] on same pole [Slirop
4 shire, Staffordshire and Worcestershire Electric Power Company].
fittings are drilled so that when turned outwards from the pole
they are readily available for service connections by the addition of
porcelain pulley insulators.
No continuous overhead earth wire is used, the H.V. ironwork
54 OVERHEAD POWER LINES
being connected to an earth plate at every pole. The earth i
nccting wire is covered with creosoted wood casing for a distanc
9 feet /Tom the ground. Earthing bows are fitted on all pole
The L.V. ironwork is connected to the earthed neutral conclur
This is sound practice as it puts the ironwork definitely at ej
potential (or .nearly so) and although contrary to the E.G. Reg
lions aw they stand at pmsent, it has been specially approved in
instance.
11; may be pointed out that it would bo undesirable to com
the L.V. and H.V. ironwork to the same earth, a,s the earth resists
juuy not bo low enough to obviate dangerous voltages in the I
wystein in case of insulator breakdowns.
CHAPTER IV.
INSULATORS.
THE material for .overhead line insulators must possess a high diele
trie strength and insulation resistance, and the insulator should '
so shaped as to minimise concentrations of dielectric stress due
surges which might puncture the material and so render it uns<
viceable. The shape is, of course, a matter for the designer , and
the operating engineer can do is to specify a high ratio of puncti
voltage to flashover voltage. A flashover will probably cause
interruption of supply by operating the protective gear, but the sup]
can be restored immediately without the delay which is inevita"
if one or more punctured insulators have to be located.
In addition to the electrical properties mentioned above, 1
insulator must, of course, have sufficient mechanical strength
support the conductor under all weather conditions.
The material which, in this country, is considered most nea
to satisfy all the required conditions is Porcelain, the mamifact
of which has reached a very high standard. The porcelain must
absolutely vitreous throughout to render it nonhygroscopic. .
surface is glazed, not to improve the insulation, but to render diffic
the deposit of dirt which increases surface leakage, and to facilii
the washing off by rain of whatever dirt does settle. The smo
glazed surface also reduces the wear of the conductor by abras:
The body of the porcelain should be ivory white but the glazing i
be of any colour, brown being considered the best, as the insula
then form less conspicuous targets for small boys.
Porcelain insulators are standardised for H.V. lines (B.S. S
fication 1371922).
Pin Insulators, High Voltage.
ELECTRICAL DESIGN. For voltages up to 33 000, pin type
sulators are invariably used.
56
OVERHEAD POWER LINES
The various ways in which such an insulator may fail electrically
will be clear from Kg. 38 and the table of particulars which follows :
Particulars of Typical Standard 1 1 000 Volt Pin Insulator.
Inches.
British Standard Test
Voltages.
(1) Puncture thickness ....
75
108 000
(2) Arcing distance (dry) A f B + + D.
50
62000
f (3) Arcing distance (wet) a  b  c .
225
39000
(4) Leakage distance ....
70
If the insulator is correctly proportioned with regard to wet and
dry arcing distances, the
leakage will be negligible.
MECHANICAL DESIGN.
Two strengths are
standardised, viz. : 400
Ib. and 800 Ib. As manu
factured, the insulator it
self is generally suitable
for 800 Ib. and the
strength is determined by
the size of the pin. He
ference to Table I. shows
that the 400 Ib. design is
suitable for supporting
poles up to a span length
of about 400 feet. The
800 Ib. insulator comes in
for longer spans and also
for angles, where the
lateral loading includes an
appreciable component of
the longitudinal tension in
the conductors as well as
the lateral wind pressure.
Pin insulators are not
Fid. 38. 11 000 volts porcelain insulator,
Leakage distance = 7 ins.
Dry spark over distance A + B + CjD = 5in
Wet spark over distance a + b + c = 2 ins,
Puncture thickness = f in.
INSULATORS
57
often used at terminal poles, but they are quite suitable for some
of the smaller conductors.
It is important that there should be no appreciable deflection of
the insulator pin under load, to avoid possible fracture of the por
celain. The B.S. Specification above referred to lays down a V of
$ of 2 based on the yield point.
V/ra
V/ORKIN& 33 K\/
PAIN TELOT 84
DRY 3PARK OVEB TEST 125 KV
PUNCTURE TEST 220 
WORKING LOAD 400 Iba
WEIGHTS . INSULATOR 15k
w.
R.T
D.30.T 62"
P.T. 108
WL. 400 Ibs.
Wra a%3
2fc Ifo Wra
PiG. 39. Typical high voltage porcelain insulators and pins.
Fig. 39 shows a series of B.E.S.A. Standard high voltage por
celain insulators. It is to be observed, however, that trouble has
been experienced with these insulators in very exposed positions,
particularly near the sea, and it is the practi.ce of some engineers to
use an insulator one grade higher than the British Standard rating.
58
OVERHEAD POWER LINES
For example, an 11 000 volt Standard insulator would be used on a
6 600 volt system. Tt is understood that B.S.S. 137 will shortly be
revised.
Loids on Insulators at Angles. First consider a straight
line pole (Fig. 40 (a)).
I
fir
Fia. 40 (a).
For simplicity, assume the whole of the load on the conductor
to be horizontal and equal to P lb. per span.. If the tangent to the
conductor at the point of support makes an angle oc with the direction
of the line, the lateral load on the insulator P = 2T sin oc lb.,
T being the tension in the conductor. The longitudinal forces
balance.
FIG, 40(6).
Now suppose the span BG to swing round through 6 degrees (Fig
40(6)), then the resultant horizontal force on the insulator = P 1
/ f)
=; 2T cos j8 = 2T sin ( oc + 
\ a
2T sin oc cos  f 2T cos oc sin
2
= P cos + 2T sin  cos oc.
2i 2
INSULATORS 59
In all practical cases the angle oc is in a plane inclined to the
horizontal, as explained in Chapter II., but the reasoning still holds
if it be remembered that T is actually due to the weight of
wire + ice loading, as well as to wind pressure. '
Now tan oc = and is of the order r^.
Ju LJ 100
.. tan cc = 04 and cos oc 999. The effect of the angle cc 011
the result is therefore negligible.
a
If 8 = 45, cos = 925, and with the smallest permissible
copper conductor on a 200 feet span, P = 122 Ib. on high voltage
lines and T m 633 Ib.
Substituting these values in the above formula we get
n
(i) Neglecting cos , P l = 606 Ib.
2i
(ii) Including cos , P : 597 Ib.
The neglect of cos therefore introduces an error of 15 L
2i *f
on the safe side.
We may then with sufficient accuracy for practical purposes omit
a
the factors cos  and cos oc and write :
2
RESULTANT HORIZONTAL LOAD ON INSULATOR
Values of 2 sin  are plotted in Fig. 41 .
/!
As a matter of fact this formula gives pessimistic values not only
because of the omission of the two factors referred to above, but
also because the wind cannot blow at right angles to both spans
simultaneously.
In our example, using 3/147 copper conductor on a 250 feet
span, P = 710 X 25'0 = 1775 Ib., and T m = I 457 Ib. If is the
maximum deviation permissible, we have
60 OVERHEAD POWER LINES
For the 400 Ib. pin.
400 = 1775 4 2 . 1457 sin ,
a
from wliich 2 sin.  = 153, and therefore, from Kg. 41,
7.4.
72
f0
08
06
04
02
7
20 40 60 80 100
Degree s Q
FIG, 41.
For the 800 Z6. pn.
800 = 1775 4 2 1457 sin ,
whence
2 sin = 428 autl 6 = 25.
a
If tlie angle pole is double armed and two 400 Ib. pin insnlato]
are used, we may assume that they share the load equally, and ther<
fore the same angle (i.e. 25) can be negotiated as with an 800 Ib. pii
Table VI. shows the maximum, angles which can be dealt wit
by the two standard sizes of pin on the span lengths suggested i
Table XIV., page 106.
INSULATORS
61
TABLE VI. Maximum Angles for Standard H.V. Pin Insulators.
Wind Load,
Maximum
Length,
feet.
H.V. Loading
P,
Ib.
Longitudinal
Tension, T,
Ib.
400 Ib. Pin,
degrees.
800 Ib. Pin,
degrees.
162 .
200
122
633
250
650
193 .
250
157
874
160
420
3/147 .
280
199
1457
80
250
3/18 .
315
239
2 125
40
160
7/13G .
335
258
2935
25
110
7/160 .
350
291
4265
15
70
7/193 .
335
297
5 635
10
50
It will be seen from the above figures tliat the heavy loads due
to the larger conductors make it desirable to avoid small angles and
to adhere as far as possible to absolutely straight runs between
definitely strengthened angle poles with tensioning insulators.
In practice, however, these standard pins are frequently used for
larger angles than those given, although calculations show that
tensioning insulators should really be used. Immunity from trouble
in such cases is undoubtedly due to the fact that the maxinram hypo
thetical loading conditions are rarely experienced.
In this connection it may be repeated that it is often desirable
to allow somewhat larger sags than the E.G. Regulations demand in
order to keep down the values of the longitudinal stresses at angles
and terminals. This remark applies particularly to L.V. lines in
which, relatively, the spans are short and the conductors large.
v Methods of Securing Insulator to Pin. The following
methods are commonly employed :
(1) Pin screwed directly into the porcelain.
(2) Pin screwed into a metal thimble, which is cemented into
the insulator.
(3) Insulator cemented on to the pin.
The first two methods are generally to be preferred as they permit
the insulators and pins to be transported and handled separately
and of insulators being easily replaced. Moreover, the fitting is
done in the factory instead of on the job. But in the first method,
the hard unyielding joint is likely to lead to cracking of the porcelain
under temperature changes, and the sharp edges of the metal thread
are undesirable from an electrical point of view. These disadvan
tages are not serious with small insulators and low voltages and an
indiarubber or felt washer on the shoulder of the pin minimises the
( OVERHEAD POWER LINES
effects of unequal expansion of the steel pin and the porcelain i
lessens the risk of fracture when screwing on.
For large insulators at high voltages the second method is
variably used in this country (see Fig. 38,, p. 56).
The third method, that of cementing the insulator on to 
pin, makes a sound job when well done, and it may have to be
sorted to abroad. Care must be taken to use a cement that will i
act chemically on the pin so as to produce a substance which expai
and breaks the porcelain. Sulphur must on no account be used
Neat Portland Cement (B.S. Specification 121 920) is the saf
material for the purpose. As the insulators reach a very hi
temperature in the sun it is fortunate that the coefficient of them
expansion of iron, portland cement and porcelain are not very c
forent. Iron has a rather larger coefficient than the other two, 1
ifc happens that the cement has the smallest modulus of elastic
and strength, under compression which enables it to act as a cush:
between the iron and the porcelain..
The cement mixture should be in the ratio of 1 pint of wa
to 4 11). of cement, in which proportion it has a semifluid ci
fdstoncy.
Care must be taken to fix the pin centrally in the hole of the
milator and to ensure particularly that there is a layer of ceim
between the end of the pin and the bottom of the hole.
Tho cement takes at least 48 hours to set, but the insulators c
bo removed from the framework after 24 hours, if handled careful
PLANTER OF PAULS is sometimes used and appears to give sal
factory service, though not nearly so strong as cement. It has 1
advantage of setting more quickly. To prepare the mixture, rn
some ordinary carpenter's glue to the consistency usual for wo
joints and dissolve it at the rate of >}<$ pint of glue to one gall
of wator. Then make a plaster of paris mixture with the consisten
of soft putty. Although the setting commences in about 40 minut
the insulator must not be moved for 2 hours, nor placed in positi
on pole for 24 hours.
It is perhaps unnecessary to say that this cementing on shoi
not bo done in frosty weather.
v/Methods of Securing Wire to Insulator. It will
obvious that the side groove must be used at angles. Whether t
top groove or the side groove is used in the straight is a matter
opinion, but the top is most largely used.
INSULATORS
63
Bronie
Clip
Vertical
Bronze
Wedges
FIG. 42. Side groove clip.
It must be remembered that the insulator is standardised for a
pull on the side groove, and the bending moment on the pin is some
what greater when
the wire is in the top.
Moreover it is impos , ^^s \ Bronze Sheath
sible to make such a /^ L^ slit dov ynwands)
good job of the bin cl
ingin when the wire
is in the top groove.
For heavy conductors
the top groove must
be used, as the line
man cannot hold the
wire in the side
groove when making
of. It is an advan
tage for the wire to
lie in the side groove
on the pole side of the insulator as there is then less chance of
the wire falling if the insulator is broken.
The ideal binder should be strong enough to prevent the wire from
slipping to and fro
through it with every
change in temperature
due to inequality of
span lengths, but it
should allow the wire
to slip before the elastic
limit of the pin is
reached. It should
also be as flexible as
possible to prevent the
setting up of crystallisa
tion in the conductor.
The use of ME
CHANICAL CLIPS is
sometimes preferred to
the usual method of
attaching conductors to insulators by means of binding wire.
Fig. 42 illustrates a clip for the side groove and Fig. 43 one for
Slotted hexagonhead studs
with square nuts
FIG. 43. Top groove clip.
OVERHEAD TOWEJl LINES
the top. The side groove clip requires a special tool but makes
good job and is certainly a time saver. It is made with ho
zontal wedges for use when the insulators have flat upper shec
The top groove clip which is of soft copper also makes a good jc
saves time and the only tool required is a screwdriver. These rr
chanical clips (especially the top groove design) naturally cost
good deal more than binding wire, but on the other hand they reqiii
less skill and time.
The following methods of bindingin and terminating are sit
gested. Table VII. gives the lengths of binding wire required f<
the various sizes of conductor. The use of sidecutting pliers shou
be forbidden.
Side Groove. Starting with the middle of the binding wire i
point P (Fig. 4:
serve the condut
tor for a lengt
equal to the dk
meter of the nee
of the insulator.
Take the en<
which leads ol
from the top o
the line wire (cal
this end A), pas;
it round the necl
of the insulatoi
and take a rounc
turn from above
downwards, round the conductor.
Then take the other end (B], pass it round the neck of the insu
lator and take a round turn from below upwards round the conductor.
Finally, pass both ends round the neck of the insulator again
and finish off with a serving on the conductor of about 2 inches oil
each side of the insulator, end A winding from below upwards and
end B from above downwards.
If these instructions are carefully followed the turns of wire
round the neck of the insulator will not ride one upon another.
Top Groove. Divide the length of binding wire given, in Table
VII into two equal paits and lay up together to form a double wire,
leaving Z inches of single wire at each end (see Table VII. , last column),
FIG. 44. Side groove binding.
INSULATORS
65
Starting with the middle of the double binding wire at point P (Fig.
45) serve the conductor for a distance equal to the diameter of neck
of insulator plus half an inch (i.e. J inch at each end).
Twist the double wire together at each end until the bottoms of
the twists reach the
neck of the insula
tor and then pass
one wire of each
pair in a clockwise
direction round the
neck and the other
in a counter clock
wise direction.
Twist the pairs
together again
when they meet
until on bending
upwards the tops
of the twists just
reach the conduc
x nr FIG. 45. Top groove binding.
(These two wires must go round the conductor in the same direc
tion.)
Now take 4 or 5 turns round the conductor with the short wire
of each pair.
Finally, take the other wire of each pair, pass them over the top
of the insulator so
that they cross
each other and the
conductor, and
A finish off with 7.
or 8 turns round
the conductor.
v. Terminating
Small Conduc
tors (Fig. 46).
Pass the conductor
round the neck of
the insulator and lay up the free end along the line part for 3 inches
(for wires below 162 inch diameter, 2 inches will do). The bends in
5
Not less than 6 ins
FIG. 46. Small conductor termination.
66 OVERHEAD POWER LINES
TABLE VII. Lengths of Binding Wire Required, for Copper
Conductors.
Diameter
of Nock of
Size of
Binding
Length of Bindinp; Wire Enquired.
Overlap when
Layln'jf up
Conductor.
Conductor.
Insulator
or Sliackle.
Wire,
S.W.Cf.
Termination.
Side.
Top.
Binder.
K.
Ins.
Ins.
Ft. Ins.
Ft. Ins.
Ft. Ins.
TllH.
.136
136
3
14
4
8
7
7
162
162
3
14
6
8
7 6
7
193
193
3
14
7
9
8
8
3/147
317
3
14
9 G
11
11
10
3/18
388
3
12
10
11
11
12
7/136
408
3
12
,
11
11
12
7/166
498
3
12
.
12
13
14
7/193
579
3
12
c
13
14
10
Approximate lengths of 1 Ib. of Copper Binding Wire are : 12 S.W.Q., 30 feet ;
14 S.W.G., 50 feet.
The lengths naturally vary with the size of the insulator, and the exact figure
should be determined by trial in particular cases, but the Table will assist when
estimating. The table allows for a layer of binding wire on the conductor itself
where in the groove, to prevent chafing between the wire and tho nock of the
insulator. This is considered good practice in this country, and incidentally it luiljw
to prevent burning of the conductor when a "flashover" occurs. Alternatively
copper tape may be used as a chafer, in which case some 20 to 30 % ICH.H binding
wire will be required.
the conductor at the point A must not be too sharp. Then bond
out the free end at right angles, leaving sufficient length (not less
than 6 inches) for connection to leadingin cable.
Now pass a length of binding wire round the insulator, twist the
two ends together and then, with the double binding wire, bind the
free end of the conductor to the line part for the 3 inches overlap,
finishing of by serving the single conductor for a length of 1 inches.
^Terminating Large Conductors. The common methods
employed are shown in Fig. 47.
H. V. Tensioning Insulators. Pour strengths of tensioning
insulators are standardised for H.V. lines, viz. 400, 800, 1 200 and
2 400 lb. } but they can be obtained for loads up to 10 tons if
required in special cases.
Figure 47 illustrates four distinct types which are in use.
Type " a "the SHACKLE Insulator is good mechanically and is
quite suitable for L.V. work. It is also used on H.V. work for volt
ages up to 6 600 volt, but above that it becomes unwieldy in size.
Type "6," the HEWLETT INTERLINKED type has been exten
sively used in the past and is a good design mechanically.
INSULATORS
67
Type " c " is also an INTERLINKED type similar to type " b,"
but it is claimed that the porcelain is shaped so as to cause a better
distribution of the dielectric stress.
SCALE
(a) SHACKLE TYPE 6.600 VOLTS , BOO POUNDS.
(6) HEWLETT INTERLINKED DISC TYPE 1 1,000 VOLTS.
1400 POUNDS (TWO 6600 VOLT UNITS)
(0) TWISS INTERLINKED DISC TYPE 1 1 000 VOLTS,
1400 POUNDS (TWO 6600 VOLT UNITS)
(d) CAP AND PIN (OR METAL HOOD) TYPE,
gZOOO VOLTS. 2800 POUNDS (TWO 1 1000 VOLT UNITS)
FIG. 47. Typical high voltage tonsioning insulators.
No cement is used in either of the above types and the porcelain
is in compression. Moreover, if the insulator breaks, the wire is
still linked mechanically and therefore does not fall. For these
reasons some engineers prefer them to
68 OVERHEAD POWER LINES
Type " d." This type, which is called the METAL HOODED, or CA:
AND PIN, TYPE, will be seen to have the porcelain in tension and th<
cement in shear, and the design is a radical departure from that whicl
was rigidly adhered to in the early days, when the porcelain was nse<
in compression only. But the porcelain of today is more uniformly
reliable than it was in the past, and this design has been used sue
cessfully for some years. Recent improvements in the methods o
fixing the pin, e.g. the " split ring " method, have enabled thi
type to be manufactured for working loads up to 8 tons. It i
certainly by far the best design for very high voltages owing to tin
uniform dielectric stress distribution.
However, for the moderate high voltages used in distributioi
work, the types mentioned may be considered to be equally reliabl
and a choice made on the basis of first cost.
./Insulators of Materials other than Porcelain. Ex
perience justifies the opinion that British porcelain has no supeiio
as an overhead line insulator and it is now strongly fortified by
B.S. Specification.
But GLASS cannot be entirely neglected. It has been and sti
is largely used on the Continent, and it is much cheaper than British
porcelain.
Up to 22 000 volts at least modern continental designs of glas
insulator appear to be thoroughly reliable, and it is a pity that th
manufacture of glass suitable for H.V. Insulators has not bee:
seriously undertaken in this country.
But both porcelain and glass are very fragile, and it is probabl
true to say that more line breakdowns are due to broken or defectiv
insulators than to all other causes put together.
Owing to this drawback, many attempts have been made t
produce a satisfactory substitute. Among such substitutes whic!
have been placed on the market may be mentioned KALANIT:
(Callenrlers Cable and Construction Co.), TELENDUKON (Thoma
De La Rue & Co.) and EBONESTOS (Ebonestos Insulators, Ltd/
All these materials are tough and nonhygroscopic, and initially
at any rate, they have the requisite dielectric strength. They appea
to give satisfaction on telegraph and telephone lines and on powe
lines up to about 6 000 volts, but they are more expensive tha
porcelain.
STEATITE, a naturally occurring magnesium silicate, has electric*
properties equal to those of porcelain and very much higher ten si]
and bending strengths.
INSULATORS 69
Insulators of this material can "be made absolutely puncture
proof. They are reputed to stand up well to stone throwing, but
at present their high cost limits their use to special situations.
Another material which shows promise is FUSED BASALT, a
darkcoloured rock of volcanic origin which can be moulded to almost
any desired shape at a temperature of 2 300 F.
It is claimed to have all the advantages of porcelain, and in ad
dition it has a strength of about 18 tons per square inch both in
tension and compression, and possesses the remarkable property of
resealmg itself when punctured. Moreover, it is said to be cheaper
than porcelain. It is understood that a number of insulators of
this material are on trial in this country.
In the present state of development none of these materials
can be recommended to replace porcelain except steatite, but they
might be tried in sections of a line subject to trouble from stone
throwing.
There is a fortune awaiting the inventor of a material which has
all the electrical advantages of porcelain, with its durability but
without its fragility.
70
CHAPTER V.
CROSS ARMS AND INSULATOR BRACKETS.
THE methods of supporting the insulators and securing them to
the pole afford much scope for ingenuity.
It is axiomatic that the poles should be cut and drilled as little
as possible after creosoting. The sapwood only absorbs the creosote
impregnation, and it is therefore important to avoid penetrating the
heartwood when cutting slots. All slots, holes, etc., cut in poles
should be painted with a hot creosote tar mixture (2 creosote,
1 coal tar).
Table VIII. gives particulars of some useful angle and channel
sections.
For pin insulators, the channel section with the web ^ertical is
better than the angle owing to the greater depth for securing the
pin, but the angle is much stronger, weight for weight, in the direction
of the line, and can be adapted for the fitting of standard insulator
pins by inserting an oak block.
Power engineers do not favour the use of timber cross arms, but
their prejudice appears to be unjustified when one considers the
fifty years' experience of the Post Office with arms of oak and other
hardwoods. Wood impregnated with bakelite varnish, known as
BAKELISED WOOD, has recently been introduced by a French firm.
It is stated to have a tensile strength three times that of the untreated
wood. and a very high dielectric strength. It appears to be a very
promising material to use for cross arms and insulator pins (and
possibly for the insulators themselves) in situations near the coast
where porcelain insulators give trouble due to the saltladen air
when fitted to steel pins and pole fittings.
All pole ironwork should be galvanised when possible. After
' immersion for twenty minutes in a solution of hydrochloric acid in
' water (equal parts acid and water) the parts should be immersed in a
' bath of pure molten zinc, and then placed at an angle to set.
When ironwork is made up locally galvanising may not always
CROSS ARMS AND INSULATOR BRACKETS 71
TABLE VIII. Particulars of Steel Sections.
Material.
Section.
Weight
per ft.
Ib.
Area
A
sq. in.
Moment
of
Inertia,
J.
Strength
ModuluSj
Z.
Uaclius
of
Gyration,
Jc.
*2 X 1 if 1
1
*2 X 1J E J
384
1125
/ 024
(^ 065
025
065
0465
0760
3 X IJ [t 1
3 V . 1 1 l___^. j
460
1352
( 0261
 1823
0255
1215
0439
1161
S
X i* 1"
*3J X 2 . [T \
r /
675
1986
{ 0713
^ 3701
0526
2115
0599
1370
z
*3 X 2 "t
o
4 X 2 [T 
4v> O _._l__._ *^
709
2085
( 0703
\ 5063
0502
2532
0581
1558
X ^ j
5 X 2 [T 1
1022 V
3006
/ 1641
 11873
095
4749
0739
1987
5 X 2 \
6 X 3 [T 
1241
3650
/ 2825
[21271
1339
709
0880
2414
X 3 LI.
itxHxt r i
234
687
f 134
1 056
128
441
286
14 x ij x i JT ]
I
If X If X 3 ft^ 
326
960
f "255
108
21
515
335
If X If X 3 / T : J
L
C5
2 X 2 X 3 ft" 
2x2 X 3 X" >
377
1110
f 392
j 164
28
594
384
1
2 X 2i X 375 fj~ ^
590
1735
/ 962
1 402
55
745
482
>
2J X 2J X 375 / p Ji ' j
L
W
3 X 3 X 375 it" 1
717
2110
f 172
\ 712
81
903
581
3 X 3 X 375 ,.f~ J
I
4 X 4 X 425 [j ^
1095
3220
f 4752
1 1953
1603
1215
779
4 X 4 X 425 X" *
I
The lesser values of J and fc must be used for struts. With, the exception of those
starred, the above particulars have been, extracted by permission of the British
Engineering Standards Association from British Standard Specification No. 6 for
rolled steel sections for structural purposes. (See p. vii.)
72 OVERHEAD POWER LINES
be practicable, in which case the parts should be immersed in
bath of hot gas tar (300 F.) thinned with a little gas oil, until t
iron attains the temperature of the bath, and then placed at an an
to set. On erection a couple of coats of tar varnish or bitumasl
solution should be given.
For simple rectangular sections, if b = breadth and d the depth,
=~ =
12 6
For circular sections of diameter D,
, TiD 4 TrD 3 ,
J== 6T Z  32 and
Eccentric Loading. For eccentric loading producing con
bined bending and direct stress, if T and C represent the direc
tensile and compressive loads in lb., M the bending moment i
Ib.inches, Z the strength modulus in inch units and A the area c
section in sq. ins., then the following formula will give the maximun
tensile and compressive stresses in the lb./ sq. inch,
(i) When DIRECT STEESS is TENSILE,
T M
,____
Jc ~Z c A"
(ii) When DIEECT STEESS is COMPRESSIVE.
f^+^
Jc ~~ A ^ Z e
__M __C
Jt ~ Z t A'
In the absence of precise information, the following values for
ultimate stresses, etc., of ordinary mild steel, Norwegian red fir and
oak may be assumed in calculations.
Special steel may be obtained of more than double the strength
of ordinary mild steel, and is generally preferred for insulator pins.
The figures for timber are good average values.
CROSS ARMS AND INSULATOR BRACKETS 73
TABLE IX. Ultimate Strengths of Mild Steel* Oak, and Red Fir.
MILD STEEL.
(i) Ultimate Tensile Stress, Tensile . 65000 Ib. /sq. in.
(ii) Compressive 55 000 ,,
(iii) Shear . . 50000
(iv) Elastic Limit, Tensile or Compressive 36 000 ,,
(v) Shear . . . 27000
(vi) Modulus of Rupture (Bending) . . 60 000
(vii) Elasticity, E . . 30 X 10"
ENGLISH OAK.
(i) Ultimate Stress, Tensile
(a) Parallel to grain ... 10 000
(b) Perpendicular to grain . . 900 ,,
(ii) Ultimate Stress Compressive
(a) Parallel to grain . . . 8000 ,,
(b) Perpendicular to grain . . 2 000 ,,
(iii) Ultimate Stress, Shear
(a) Parallel to grain . . . 800 ,,
(b) Perpendicular to grain . . 4000 ,,
(iv) Modulus of Rupture (Bending) . . 9 000
Elasticity, E . . 12 x 10
NORWEGIAN RED Era.
(i) Ultimate Stress, Tensile
(a) Parallel to grain . . . 8 000
(b) Perpendicular to grain . . 500 ,.
(ii) Ultimate Stress Compressive
(a) Parallel to grain ... 6 000
(b) Perpendicular to grain . . 1 500
(iii) Ultimate Stress Shear
(a) Parallel to grain . . . 500
(b) Perpendicular to grain . . 4 000
(iv) Modulus of Rupture (Bending) . 7 800
(v) Elasticity, E . . . 12 x 10
* Although referred to as " iron " in the colloquial sense in various places
throughout the book, " Mild Steel " (2 % carbon) is the material most commonly
used in overhead line work.
Good quality wrought iron, however, is very little inferior in strength to mild
steel.
Selections from the many types of pole ironwork met with, are
shown in Figs. 26 to 37, and to assist the reader to form opinions as
to their respective merits and demerits some rough calculations are
given.
In practice it may not be usual to calculate very closely, neither
is it possible to be very precise, but from an inspection of some types
of fitting in use, the writer is of opinion that there is sometimes a
lack of appreciation of the stresses to be dealt with and that it. is
fortunate for the engineer concerned that the hypothetical loading
conditions are seldom experienced.
74 OVERHEAD POWER LINES
1 The importance of preventing deformation of the ironwork und
stress must be emphasised. Any appreciable movement will alt
the stress distribution in the insulator and may lead to fracture
the porcelain. The designs are checked on the basis of the maximu
(lateral) horizontal and vertical loadings, but there may always 1
a certain amount of unbalanced longitudinal pull. The latter mi
reach large values at angles since, as mentioned before, the wir
cannot be at right angles to both spans simultaneously. Moreove
reversals of stress occur when the direction of the wind changes ar
the fitting of struts, ties and bolts is never perfect.
Eor the above reasons it is desirable to keep the working stressi
low. It is proposed to allow a factor of safety of 25 on the elast
limit of iron and steel in tension, shear and bending and on the cri]
pling load for struts. Fo&the timber a factor of safety of 35 on tl
ultimate stress will be taken. All struts will be assumed to be hinge
at both ends, although this may appear to be pessimistic in some case
In accordance with the above the Working Stresses will be take
as follows :
3
Working Stresses in Mild Steel.
I. DIRECT TENSION AND BENDING. Safe Working Tensile Stres
. 36000
=^/< ^ 7TF = U 40 lb ' / SC I m '
jL'O
II. DIEEGT COMPRESSION (STEUTS). In the following I = effec
tive length of strut in inches ; <7 = moment of inertia in inch units
"k = radius of gyration in inches ; A = area of crosssection i:
square inches.
(a) For values o/y > 100.
K
Euler's formula for a strut hinged at both ends will be used.
7T
n v i i D
Crippling load : B =
J = AW.
Safe Working Compressive Stress
/  10 30 1Q6 A7 2  12 x 1Q7
/c ~ I* . 25 . A
(b) For values of  from 60 to 100.
CROSS ARMS AND INSULATOR BRACKETS 75
The following empirical straight line formula will be used (see
B.S. Spec. No. 61924) :
Crippling Stress =/ c = (46 000  166 7 ) Ib. / sq. in.
\ KJ
.. Safe Working Compressive Stress = l8 400 66 Ib. / sq. in.
(c) For values ofj < 60.
1
When j = 607, the straight line formula gives / c = 14 400
A*
Ib. / sq. in., which is the maximum value allowed in tension.
Now it is contrary to experience to find members stronger in
compression than in tension ; so for short struts f c will be limited.
to 14 400 Ib. / sq. in., although the straight line formula will indicate
a larger value.
III. SHEAR. Safe Working Shear Stress
Ib. / sq. in.
Working Stresses in Bolts.
27000
25
= 10800
TABLE X. Particulars of Bolts (WMtworth Threads),
Overall Diameter,
ir.s.
Total Area,
sq. ins.
Diameter at Bottom
of Threads,
ins.
Area at Bottom of
Threads,
B<I. ins.
5
196
393
121
625
307
509
204
75
442
622
304
875
601
733
422
100
785
840
554
It is recommended that the Factor of Safety for bolts should be
25, also based on the elastic limit, since although the elastic defor
mation of bolts may be negligibly small as far as the shape of the
fitting is concerned, there is always an indefinite tensile and , tor
sional stress in the bolts due to tightening up the nut.
The tensile load might easily be 3 000 Ib. which means in a finch
bolt a tensile stress of = 9 900 Ib. / sq. in. at bottom of threads.
*o04
The stress may be 1520 % greater than this due to torsion.
The necessity for large washers on bolts through timber will be
apparent.
76 OVERHEAD POWER LINES
The bearing pressure under washers may be taken as 25 % greater
than the greatest permissible compressive stress allowed in the
general design, e.g. if the tensile load on the bolt is 3 000 lb., the
area of washer should be
3 000
= 56 sq. ins.
1 500
A 3 in. X 2 in. X \ in. washer will meet the case.
Bolts through poles should always be a driving fit. To ensure
this the hole should be drilled with an auger TIT inch only larger than
the bolt.
Coach Screws. Coach screws are found to be very useful in
pole line work when used intelligently.
To fix the screw a hole should be bored a sixteenth less than the
overall diameter of the threaded portion of the shank, and if an
appreciable part of the unthreaded portion of the shank penetrates
the pole provision should be made for it by enlarging the hole to
the requisite depth.
Driving the screw part of the way with a hammer, as is frequently
done, always lessens the holding power.
A number of experiments have been made to test the holding
power of coach screws. Allowing a F. of S. of 35 to 4 and rounding
off the figures the safe holding power when inserted across the grain
in Norwegian Bed Fir, per inch of penetration of thread may be
taken to be :  inch diameter, 300 lb. ; inch, 350 lb. ; f inch, 400 lb.
These figures apply to any direction of pull, providing that the
coach screw penetrates the pole up to a point not less than f inch
under the head. That is to say, the thickness of the material secured
to the pole must not exceed f inch . If it does the holding power is
reduced when the pull is nonaxial, although there may be the same
length of penetration of thread into the pole.
Working Stresses in Norwegian Red Fir. The elastic
limit for timber is approximately 75 % of the ultimate stress, but
there is not the same precision about the figure that there is about
the elastic limit of steel. It is proposed to base the Factor of Safety
on the ultimate stress in all cases.
Bearing Pressure of Round Bolts on Timber at Bolt
Holes.
I. LOAD AT EIGHT ANGLES TO FIBRES. In this case the bearing
area may be based on the full diameter of bolt (d).
CROSS ARMS AND INSULATOR BRACKETS 77
II. LOAD PARALLEL TO FIBRES. In this case it can be shown
that the effective width of bearing surface is only about sixtenths
of the diameter (i.e. Q'6d), e.g. with a finch bolt, the safe maximum
working load per inch length of bolt
r , 1 500 X 75
In case I. = ^= = 321 ID.
o'O
, TT 6 000 X 75 X 6 ,
In case II. = ^= = 771 Ib.
o5
It might be noted in passing that in Case II. there is a load at
right angles to the fibres equal to about onetenth of the longitudinal
load. This transverse load tends to split the pole and has to be
taken into account in designing timber joints, but it is not likely to
be of importance in connection with pole fittings.
Case III. LOAD INCLINED TO FIBRES. In this case the simp
lest procedure is to resolve into two components parallel and at
right angles respectively to the fibres.
Continuing with the finch bolt, if the load per inch length = P at
an angle of 30 with the fibres, then the component along fibres =
P cos 30 and at right angles thereto = P sin 30.
771
As far as longitudinal strength is concerned P may be ^r =
771 ^91 391
4^~ = 890 Ib., but it is limited to 4^ = = = 642 Ib. by the
866 sin 30 5 J
transverse strength.
We will now consider the various fittings illustrated :
In the following, W = dead weight of wire plus ice ; P^ = P f
/ f)\
T m (2 sin  ) = P) T m oc, in which P l = total lateral load on
\ 2/
insulator, P lateral load due to wind pressure, T m = maximum
safe tensile load on conductor, 6 = angle of deviation of line and
n
cc = 2 sin  which is plotted in Fig. 41, page 60.
2i
Table XL gives values of W and P for the standard lengths of
span suggested in Table XIV., page 106, which will be kept in mind
throughout, as well as Table VI., page 61, giving the maximum
deviations in the line which are permissible, using standard insulator
pins.
78
OVERHEAD POWER LINES
TABLE XI. Values of W, P and T m for Standard Spans erected to
E.G. Regulations for H.V. Lines.
Conductor.
Standard
Span,
feet.
TF per foot.
Ib.
P per foot,
( Ib.
TF (total),
Ib.
P (total),
Ib.
T ,
m'
Ib.
162
200
331
608
66
122
633
193
250
379
629
95
157
874
3 / 147
280
521
710
146
199
1457
3/18
315
654
758
206
239
2125
7/136
335
762
771
255
258
2935
7/166
350
999
832
350
291
4265
7 / 193
335
1244
885
417
297
5635
Fig. 26 (page 45).
Cross Arm. Suppose wind to be blowing from left to right
(Fig. 26 (a)) and assume bending at section " aa " at centre of arm
(i.e. neglect support given by slot). First consider the two sides
independently.
(a) Right Side. It is a case of eccentric loading
A = 2 085 sq. ins., Z t ~= 2 532 inch units (Table VIII., page 71),
M=24F+8P 1 .
3
Channel
a*
W
FIG. 26 (a).
Maximum value of P x = 800 Ib. when the stronger standard pin
is used, therefore for 7 / 193 conductor on standard 335 feet span
ft
800
24 x 417 + 8 x 800
2 085 ^ 2 532
= 380+6 480 = 6 860 Ib. / sq. in. ,.
= stress at outer edge of top flange.
Similarly /,, the stress at outer edge of bottom flange,
380 = 6 100 Ib. / sq. in.
6480
CROSS ARMS AND INSULATOR BRACKETS
'(&) LeftSide.
79
.
A
z;
24F = 6 400
10 000 =  3 600
= 380 1 420 = 1 040 Ib. / sq. in.,
i.e. the stress at the outer edge of the top flange is tensile and equal
to 1 040 Ib. / sq. in. At the outer edge of the bottom flange
f c jfc= 380 + 1 420 = 1 800 Ib. / sq. in.
We have, therefore,
Maximum tensile stress in top flange = 6 860 Ib. / sq. in.
and maximum compressive stress in bottom flange = 6 100 Ib. / sq. in.
These stresses are well below 14 400 Ib. / sq. in., the maximum safe
working value.
The maximum unbalanced tensile stress in the top flange = 6 860
 1 040 = 5 820 Ib. / sq. in. = 5 060 + 760 = 8 * 2 f gL* 2 + 380
X 2, and the unbalanced compressive stress in the bottom flange =
61001800 = 4300=5060  760 == 8 X 8 ^ X 2  380 X 2.
a* Qua
The bending moment due to these forces (viz. 5 060 X 2 532 =
12 800 Ib.ins.) must be dealt with by the slot.
Strength of Slot (Fig. 26 (&)). Assume pole diameter D= 8
ins. and depth of slot b = 1 ins.
4 43
Area of segment = ^bd = Q X ^ X 312 = 624 sq. ins.
80 OVERHEAD POWER LINES
Since 6 < the arc may be assumed to be a parabola for whicl
2
j;*w.
and
Now tlie maximum compressive stress on the timber in the slot
for the values of If and P t being considered,,
2x 417 12800 O.OAIT, /
= 6^+M = 342 lb  /
but/ c must not exceed 1 715 Ib. / sq. in.
* .. P! must be reduced from 800
Strength of Arm Bolt Fixing (Kg. 26 (c)). The bolt pivots
about the centre of the 6 inch of pole which remains after the 11
inch slot has been cut.
If f c is the maximum compressive stress in the timber due to the
bending moment on the bolt, we have
Bolt dia. = 075 inch ; .. A 65 X~75 = 488 sq. ins. ; G = 2Pj Ib.,
and M = P x X 2 X (325 j 12) = 674?!,
(If a 3 in. X 3 in. oak arm were used
M = P! X 2 X (325 + 15) = 95P x
and the method of fixing therefore much weaker.)
The compressive load diagram is shown in Fig. 26 (c).
The total load on timber on each side of axis, if / B = maximum,
stress due to bending moment, = 325 x 75 X ~ 1 217 / 6 , */
a
CROSS ARMS AND INSULATOR BRACKETS 81
The centre of pressure of this triangular load
2
= 325 X = 216 inches from axis.
A
.. Moment of resistance = 1217 X 216 X 2 X / & = 525/ 6 .
Whence Z = 525 inch units.
Substituting known values in the stress equation we have
2P X 674P!
~~~~ J n"r ~T~ P SI? ""~~~ 1'
1500
35
429 Ib. / sq. in.
/ at right angles to fibres must not exceed
.. P l is limited to = 252 Ib. as far as the timber is concerned.
Now, consider the bolt.
Fia. 26 (c).
The safe moment of resistance of a finch bolt, if we base the
F. of S. upon the elastic limit
_^3^ _ OT
. 14 400 = 597 Ib.ins,.
32
32.64
Assuming no flexure of the bolt, the maximum bending moment
on it occurs at the centre and is equal to half the total bending
moment,
.e.
, = X 252 = 849 Ib.ins.
As this is too great for the bolt, the total bending moment must
be reduced to 597 + 849 = 1 446 Ib.ins., and P 1
1446
to
674
= 215 Ib.
82 OVERHEAD POWER LINES
But tliis is a pessimistic result, since if tlie nut is tightened up
so as to produce a tension of 3 000 Ib. in the bolt, the frictional force
between the arm and the pole is quite considerable.
Assuming a coefficient of friction of iron on wood of 03, this
force = 3 000 X 3 = 900 Ib., and allowing a Factor of Safety of 35
it will enable P x to be increased by x 55^ = 124 Ib., making
0*0 x & o'oi
a total of 215 + 124 = 339 Ib,
If, on the other hand, the nut is drawn up lightly we may base tlie
Factor of Safety upon the ultimate strength of the bolt and its
moment of resistance would then be 597 X . = 995 Ib.ins.
14 400
and therefore the timber will give before the bolt,
If the length of the spanner is not too great (12 to 15 times tlu
diameter of the bolt) bolts should always be drawn up as tightlj
as possible, since the frictional forces help both the timber anc
the bolt itself and so materially increases the holding power.
It will be seen from the above figures that the simple slot fixin,
is suitable for all sizes of conductor and span lengths given in Tabl
XIV., providing the line is quite straight, but for larger lateral load
such, as arc experienced at angles, some form, of reinforcement i
necessary.
Wo will first consider the effect of adding diagonals as show
dotted in Fig. 26, page 45, ' : 
Diagonals. In order that the diagonals may be effective, it
desirable that there should be no deflection at their points of attacl
ment to the cross arm.. The calculations should, therefore, real
be based upon deflections rather than upon moments, but it
simpler to consider the latter and the result so obtained is sufficient
accurate for our purpose.
Assuming the dimensions shown in Fig. 26 (d) and taking m
ments about " a," we have, if C is the compressive load in t:
right hand diagonal,
16= W X 24+ P t X 8.
Taking W = 417 Ib. and Pj. = 800 Ib., we get G = 1 450 Ib. Sir
larly for the lefthand diagonal
'JLx 16= FX 24PjX 8
andC= 318 Ib.
CROSS ARMS AND INSULATOR BRACKETS 83
Now it will be impracticable to use anything less than li X 1
X  angle, owing to the end fixing requirements. For this section
I 17
j j = 585. As this is less than 60, the safe working stress
= 14 400 Ib. / sq. in. ,
.. Max. safe working load = / x A = 14 400 X 687 = 9900 Ib.
The section is therefore amply strong enough.
In accordance with the above the load on the slot would be
upwards and equal to 1 025 + 225 417 417 = 416 Ib., but as
explained later, the load on the slot is much more likely to be
vertically downwards when the fitting has settled down.
Now consider the strength of the bolts at " a " and " de" Tak
ing " de " first, and assuming that the vertical load on the timber is
uniformly distributed, the safe working load = 1 715 X 6 X 8 X
P Boo
pfj^Boo
f =^
8"
< 8" >
* iz ^i
I rt 800
aoo ^ a
\ ' 225
f
A. nn /
\
\
12"
ffc5 ,t ^
. . "9 X\
I
//^~
\A/3 A 1 7" x
v TI / . 2,2.5 \
i*h r
/IQZS W4lT
hTHTI
"* (
225'
I025
FIG. 26 (d). ;
75 = 3 080 Ib., so there is ample strength as far as the timber is
concerned.
The maximum bending moment on the bolt (finch diameter) =
1 025 X 2125 = 2 180 Ib.ins., and its moment of resistance is
1 490 Ib.ins. at the elastic limit.
The frictional force between angle iron and pole may be assumed
to increase this by (3 000  1 025  225) X 4 X 2 4 25 =. 560
Ib.ins., making a total moment of resistance of 2 050 lb. : ins. only,
which is quite inadequate.
Actually, however, this is a pessimistic result, since although the
diagonals should be capable of dealing with the total load initially,
flexure of the lower bolt will enable the arm to take its share of the
load, and for this reason the maximum bending moment on the bolt
should not exceed about one half the value calculated above. More
over, when the bolt bends, the load distribution on the timber in the
84 OVERHEAD POWER LINES
bolt hole is altered, and instead of remaining uniform it becomes
greater near the surface of the pole. For a given vertical load, this
means a shift of the centre of pressure of the reaction towards the
point of application of the load and this reduces the bending
moment on the bolt. All things considered, a finch bolt will; be
found quite suitable for the requirements in this case.
With regard to the bolt at "a," without the diagonals the load
on this bolt would be 1 600 Ib. With the diagonals, the load is seen
,,,.. to be considerably greater, and it is there
" X 8 rap ' fore quite useless to add diagonals, without
strengthening the arm fixing, for which
& . n '
26 (e] ^ e ma ximum sate load was shown above
to be 678 Ib. only. The best way to do
this is to put a 1 strap round the pole, as shown in Fig. 26 (e).
In this way the diagonals will serve a useful purpose for loads ' up
to P x = 800 Ib. on pin insulators. Above this value, tensioning
insulators must be used and then P l acts along the axis of the arm,
thus rendering diagonals really unnecessary, although it might be
desirable to retain them. By fixing the arm to the pole by means
of a strap in this way the lateral working load at angles is limited
only by the buckling strength of the pole.
Double Arms. Double arms may sometimes be desirable at
angle poles to maintain sufficient clearances between conductor and
pole on outside of angle. In this connection, the reader may be
reminded that to maintain a spacing of sc. feet throughout the span,
the distance between insulators at angle poles must be increased to
/g
 T, 6 being as before the deviation in the line. This is one of the
reasons why " H " poles are often used at large angles, another being
the limit imposed by the buckling stress of the pole.
Assuming finch bolt and 8inch pole as before and two l~inch
slots, the safe working load on the timber in the bolt hole
= 5 X 75 X 429 = 1 610 Ib.
The bending moment on the bolt with this load
For reasons explained above this may be considered satisfactory for
a value of P t of about 800 Ib.
CROSS ARMS AND INSULATOR BRACKETS
Cap Fitting (Fig. 26 (/)). It must be here pointed out that a
distance of 9 inches above the
pole, as shown in Fig. 26, is
rather on the small side. 10
inches is about the minimum
practicable figure, using stan
dard 11 000 volt pin insulators.
To simplify the considera
tions, assume a theoretical
design with the dimensions
shown in Fig. 26 (/).
(1) First consider the 'part
cdfe. The BENDING MOMENT acting on this part = 7 P Ib.ins.
approx.
The MOMENT OP RESISTANCE to deformation will be four times
that of one 2 in. X f in. section
iff X 36 000) = 4( 2X 6 9X 6 3 f ) = 4 X 1 090
\ \
4 : 42"
3
f2"
P
> 6
e.
'
1 , "'
^
(
i
i
h
: 1
%"t
: b d
FIG. 2C
\6
= 6 760 Ib.ins.
To this might reasonably be added the moment of resistance of
the two inch insulator bolts
= 1 210 Ib.
. P at elastic limit
6760 1730
(2) Now consider the other part abed. Between ab and cd the
fitting is a girder without a web, and it is, therefore, relatively weak
as far as shearing is concerned.
If the connections at e and d were pivoted, the shearing load P
acting along the line cd would produce bending moments in the two
flanges independently, and they would share the load equally if the
lower flange were supported at b. But actually the unsupported
length of the lower flange is 6 inches and of the upper 2 inches only
and as the deflections are proportional to the cubes of the lengths
it will be clear that the upper flange takes most of the load. It is,
therefore, suggested that the MOMENT or RESISTANCE to deformation
is the sum of the moments of resistance of the sections at a and o ~
2x1 690 Ib.ins. The BENDING MOMENT due to the shearing load
2P Ib.ins. .. P = 1 690 Ib.
86 OVERHEAD POWER LINES
In the practical design the effective lengths of ac and ce depend
upon the fitting of the cross strap, but the above treatment is suf
ficient to indicate roughly how the strength will be affected by alter
ing the dimensions.
Experimentally, the fitting shown in Fig. 26, with a single rivet
on each side was found to have an elastic limit of 1 300 Ib. approx.
and therefore a maximum safe working load of 520 Ib.
The following points may be noted :
(1) The rectangular shape is necessary in the example chosen in
order to take double insulators, but sloping the sides inwards as
shown in Fig. 30 makes a better job with single insulators.
(2) The cross strap should always be bent downwards where
riveted to main member so as to reduce the effective length of ac
to a minimum. Two rivets on each side placed diagonally will
enable the working load to be increased a little.
(3) Although it will slightly decrease the value of the safe working
load, the distance ce may with advantage be increased an inch or
so in order that insulator pins of the same length may be used on
both cap fittings and cross arms.
(4)  .inch bolts are shown, but 3J in. X  in. coach screws will
do equally well.
(5) The upper bolt must be at a sufficient distance from top of
pole to avoid crushing the timber. This can be checked as explained
on page 80. 4 inches is about right, but it is advisable to plane the
pole a little so as to get a fiat bearing surface 2 inches wide.
Fig. 27 (page 45).
Consider Section " aa " (Fig. 27 (a)).
A ~r Z,
_f + T TO cc lBW+1(P+T n cc)
~ A ~r Z {
A I 352 sq_. ins., Z = 255 inch units.
'Introducing values for 193 conductor on 250 feet span we get
158+_87a 18 X 95 + 7(158 + 874 oc)
/ =
'* ~
I 352 255
= 11 167 f 24 645 oc = 14 400
.. oc = 13 and 6=1 (Fig. 41).
CROSS ARMS AND INSULATOR BRACKETS 87
This fitting cannot be used for conductors larger than 193 on the
standard spans considered. It is a simple design but makes very
poor use of the material.
Fig. 28 (page 45).
We will consider the top lefthand fitting of Kg. 28 which pro
vides sufficient clearance from the pole for a 22 000 volt line.
Dimensions assumed in calculations are given in Fig. 28 (a).
The diagonal may be taken to be a strut concentrically loaded,
but the horizontal members are subjected to eccentric loading. In
this case we will base the calculations on 7 / 136 conductor in the
straight on a span length of 335 feet for which W = 255 Ib. and
P 1 258 Ib. The direct loads, as determined below, are shown in
Fig. 28.
tfia. 27 (.).
FIG. 28 (a).
Load in Diagonal.
If C = Compressive load in diagonal,
X 20 = 215F
X
(i) No WIND.
P l = 0, W = 255,
...(7= 127 X 255= 324 Ib.
(ii) WIND OUTWARDS.
C = 127 X 255 + 415 X 258
= 324 + 107 = 431 Ib,
OVERHEAD POWER LINES
(iii) WIND INWARDS.
= 324 107 = 217 Ib.
Strength of Diagonal.
I 225
= 312.
k 289 X 25
.. Maximum Safe Working Compressive Load
__ 12 x 10 7 x_A. __ 12 X 10 7 X 15 X '25
~ 312 2
= 462 Ib.
FIG. 28(0).
Load on Horizontal Members. The horizontal members
make an angle = cos" 1 . 96 with the direction of P t . It is proposed
to neglect this.
(i) No WIND.
Direct Load = C X
L 2 jf = 68TF = 68 X 255 = 173 Ib.
19
Bending Moment = l5Tf = 15 X 255 = 3825 Ib.ins.
1 _ . 7 2 2 x " 25 x 22
4 t) o
CROSS ARMS AND INSULATOR BRACKETS 89
(ii) WIND OUTWARDS.
Direct Load
19
1 = ^(324 + 107) + 258
'
= 173 + 57 + 258 = 315 + 173 = 488 Ib. (tensile).
Bending Moment = 15W j 7 PI
= 15 X 255 + 7 X 258 = 2 200 Ib.ins.
T_ M
'''^~A + Z t
488 , 2200  A0on , ,
1_ _ 7 088 Ib. / sq. m.
3
Maximum permissible Tensile Stress = 14 400 Ib. / sq. in.
(iii) WIND INWARDS.
Direct Load = 315 173 = 142 Ib. (compressive).
T
FIG. 28 (c).
Bending Moment. In this case, the members are curved, and
the maximum Bending Moments occur at the points furthest away
from the straight lines joining the two ends of the members,
i.e. M = C X 24 Ib.ins. (Fig. 28 (c)).
Z on weaker axis = 2^
o
bd* 2.2.
6
_
24'
, _ 142 142 x 24
../ c _ 1 + ^
24
= 142+8 180 = 8 322 Ib./sq. in.
(Maximum permissible Compressive Stress = 14 400 Ib. / sq. in.)
It may be noted in passing that if additional coach screws were
90 OVERHEAD POWER LINES
placed at the points nn, where the straps are tangential to the po.
they would become simple concentrically loaded struts of leng
16 inches and very much stronger to resist wind loading inwar
towards the pole.
It will be seen from the above that the fitting is not suitable f
conductors larger than 7/ 136 unless the span length is much r
duced, and it can only be used with 7/ 136 if the line is qui
straight. If the calculations are repeated for 3/18 conductor
will be found that an angle of about 6 degrees can be negotiate!
and of course, with the smaller conductors, much larger angles ca
be dealt with.
As it is the relative weakness of the diagonal which is the limitii:
factor, the fitting can be strengthened at very little cost by increasir
the section of this member. The buckling load of a 1\ inch X $ inc
section is more than double that of the 1 inch X J inch section use<
Actually, the ends of the diagonals are more nearly fixed tha
hinged and there is considerable friction between the horizonfe
members and the po]e when the diagonal has to support the greate;
load (i.e. when the wind blows outwards). Also, the diagonals ma
possibly take some of the direct load P i} which would tend to reduc
the compressive load in it when the wind blows outwards.
These factors are comforting, but should not be made use of i
the design.
Strength of Coach Screws. The coach screw supportin
the greatest load is that at the bottom of the diagonal. The vertics
19
component of the maximum load on the diagonal is 431 X 7^73 =
Zji'O
365 Ib. The length of thread on a 3inch coach screw exceed
2 inches and the safe working load is therefore at least 300 X 2 =
600 Ib. This neglects the help given by the frictional resistant
between diagonal and pole, so the holding power is ample.
This is a very sensible and cheap fitting and is easy to fix. I
may be necessary to fit bird guards in some areas. As stated before
the calculations refer to the top lefthand fitting on Fig. 28. Th<
other two fittings are somewhat stronger.
Fig. 29 (page 45).
Fig. 29 is a weak design, and will only be briefly considered,
The maximum safe MOMENT OF RESISTANCE
TT.tZ 3 / Trxl X 36000 , ,, K1 _ .
^ 3Tx^B = 32 X 25 = 1 415 lb ' ms 
CROSS ARMS AND INSULATOR BRACKETS
.93
When the wind is outwards and the conductor in the side groove
nearer the pole, the BENDING MOMENT = 65TF f 13Pj [Kg.
29 (a)].
Introducing values for the smallest conductor, 162 on a 200 feet
span we get
65 X 66 + 13 XI22 = 2 015 Ib.ins.
(The shearing and direct compressive loads on the section are
negligible compared with the bending load.)
This type is, therefore, ruled out altogether for the span lengths
under consideration. It is really an elongated insulator bolt, and
its use in H.V. work must be limited to 6 600 volt lines, with small
Channel,
FIG. 29 (a).
PIG. 30 (a).
193
conductors on short spans. It may be used with 162 and
copper on span lengths of 130150 feet in straight runs only.
For L.V. lines, in which the spans are usually short, the hypo
thetical loading conditions and the required pole clearances are
both less, the type will be found useful.
Fig. 30 (page 46).
Side Fitting. It will be found that the following calculations
(similar to those for Fig. 29 (a)) determine the strength of this
fitting. Providing that the cutting and shaping at the lower end
is reasonably well done, the strength of section is increased when
opened out.
OVERHEAD POWER LINES
Fia. 32 (a).
CROSS ARMS AND INSULATOR BRACKKTS 0tf
The MOMKNT OF RESISTANCE of .section nt the point where the
web is cut away .from the flanges
x 520  7 580 Ib.iiw.
The BENDTNO MOMENT on this section = 10 >f 157'j. [Fig.
30 (4J.
Introducing values for 7/006 on a 350 foot span, we. have, in
straight runs
10 X 350 + ]5 X 291 7 8(55 Ib.ins.
Thin is somewhat high, but as we have, been conservative as re
gards working stresn, the fitting might be considered about right.
With 3 / Id 7 on, 280 feet spans, we, have
10 X 'MO H~ J GlOO + 1 457 X 2 Bin  7 580,
\ A/
whenco ? 8,
that is, the fitting is suitable for an angular deviation in the. lino of
<S degrees. This is a simple and economical fitting.
Cap Fitting. ....... This may be cheeked HH explained on, page 85,
It is stronger than the side fitting.
The, calculations for the remaining fittings will bo left as exercises
for the rea4er.
Fig. 32 (</) shows the approximate loads on the. various members
of the, CaJ lender side pole fitting illustrated in Fig. 32 (single, insu
lator) with an outward wind load on insulator of '100 Ib, and a dead
weight load (conductor and ice) of 200 Ib.
94
CHAPTER VI.
SIMPLE WOOD SUPPORTING POLES.
THERE are three types of pole to consider, viz. (a) SUPPORTS,
(6) ANGLES, (c) TERMINALS. SUPPORTS are poles on which there
are normally no longitudinal forces and which have, therefore,
merely to support the wires and resist lateral loading due to wind
pressure. Unbalanced forces at angles and terminals are dealt with
by stays and struts. Single wood poles will be found suitable for
most of the lines under consideration and attention will be confined
to them in this chapter.
Poles of NORWEGIAN RED FIR are standardised (B.S. Spec. 189
1921) and used very largely in this country. If felled at the correct
season of the year and properly seasoned and
creosoted they have a useful life of upwards
of 40 years. It is specified that the polos
shall retain their natural butt.
' The Regulations, however, do not pre
clude the use of other species of timber, and
in this connection the " Cobra " method of
impregnation (Cobra Limited, 30 Norfolk
Street, Strand, W.C. 2) may be referred to
as it can be applied to almost any kind of
timber and to standing poles as well as to
new poles before erection. The preservative
is a water solution of 85 % sodium fluoride
and 15 % sodium dinitrophenate and it is injected by puncturing
the pole at various points on its circumference by a special^
apparatus.
To Determine the Size of Pole Required.
Notation (see Fig. 48).
L = Overall length of pole.
H = Height from ground to point of loading (the point of loading
is the centre of pressure due to wind load on all the con
ductors and wires carried on the pole).
H
V ft
FIG. 48;
,D
SIMPLE WOOD SUPPORTING POLES 95
A = Length of pole above point of loading.
Ti = Length of pole buried in ground.
D = Diameter of pole at ground level.
All the above dimensions in inches.
P = Wind load on wires in pounds.
The BENDING MOMENT on the pole at ground level due to wind
load on wires
The WIND LOAD ON POLE (neglecting taper)
_ _D(H + A)8
P 144 LD >
H 4 A
which may be assumed to act at a height of = inches.
2i
BENDING MOMENT DUE TO WIND LOAD ON POLE
D . (R + A} z . 8 .
= 2.144 lb  1M 
TOTAL BENDING MOMENT
2 . 144
The MOMENT OE RESISTANCE of a circular section
/, the MODULUS OP KUPTURE, has been found experimentally to
have an average value of 7 800 Ib. per square inch for RED FIR. .
Equating BENDING MOMENT to MOMENT OE RESISTANCE and
allowing a Factor of Safety of 35 as required by the E.G. Regulations
we have
TlD 3 7800 ^prr, D (H f A) 2 8
p 32 ' 35 + 2 144 '
: Whence P = 219^ _ I ^ . (H + A}\
ti dO XI
All dimensions in inches.
Prepared from this formula, Fig. 49 gives values for the maximum
permissible loading for poles from 6 to 14 inches in diameter and of
;'i
96 OVERHEAD POWER LINES
lengths from 28 to 45 feet, assuming A = 2 feet. When P and //
are known, the theoretically correct size of pole can be determined
from this figure and the nearest standard, size then selected from
Table XII.
<o
SIMPLE WOOD SUPPORTING POLES
1
irf^
1 j 1 [ f O O 10 O 10 O" 10 l2 [2 o IO IO (=3
' ' ' ' 1^ OO O3 O C<l ^ 1 O5 N CO rH 1O O IO H
i I rl IH rl i I M (M CO CO <il rP a!
!ii
' 1
f III! "J** 1 H^H'M J^ l^t<r<I<NlHl MjTHW)^ j
1 J I I 1 o i i 1 1 i <M oo co t^ ia o i i~ oo o j tiD
psSS
i
3
Cf>
P
^ 5 2
,H , * **** H. ***. 1
&
M ?
rd
f?
= .
"S
p
P .5"" 1
'S'P
P p
"o
fl
1 "a <"
CO O <N rh CO 00 O CM ^ CO O >O O 10 O IO O JO O IO .H a?
r < C^l C< C^ C^l C4 CO CO CO CO sj^ ^ IO &O CO CO I'* I"* OO OO i Q_j
I 'ffl to
Ill's
"3 W
10 10 10 o P Sw ^
as
' ' ' cb CO ij 1 * ib CO t CO CD Ol ^ t^ cb I ' PH m
rM iH I I rH Ol t>
n''?
I
' 2>'o
1 i 1 oo co co co 05 as OS'OT o ii' C^CO'TH  [ 
4
prt,2
rl rl rl iH rt i^ F
w.fci
S
i
a
& !
H ..g
"JR
1 1 rt rrt
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98 OVERHEAD POWER LINES
It will be noted that the wind pressure on the pole arms, brackets
and insulators is neglected. This is always relatively small and
is largely compensated for by assuming the pole bo be. of uniform
diameter, whereas it actually has a taper of about 1 in 100. The
assumption that the pole would break at the ground level is close
enough for practical purposes. This is where decay sets in, al
though it may not be theoretically the weakest section in a new
pole. The standard poles are rated on their minimum diameter
at a point 5 feet from the butt. The section considered above is
at a distance from the butt equal to about onesixth of the overall ,
length, i.e. for the lengths most commonly used, distances from 45
to 65 feet. As the taper is only 1 in 100, no' appreciable error is
introduced. ..
" A" the length of pole above the equivalent point of loading
(i.e. the centre of pressure of the wind loads on all the wires) is
frequently greater than the 2 feet assumed. The error introduced i
by this difference is small for span lengths up to 350 feet or so with
 the most common arrangements of conductors.
Example. The wind pressure on a single 3 / 147 (05 sq. in.) ice
covered conductor (f in. ice) = 0710 Ib. per foot run. With four
such conductors on a 250 feet span, the total horizontal lateral load
on the pole due to wind pressure on the wires = 071 X 250 X
4 = 710 Ib. This assumes that the diameter of the earth wire is
equal to that of the conductors, which is near enough for practical I
purposes.
Note in Table I. that the wind loading does not vary more than >
45 % over the whole range of conductors given. In this case, if a
T V galvanised steel earth wire were used, the wind load would be ;
157 Ib. only instead of 1775 assumed in these calculations.
The sag at 122 F. = 402 feet (Fig. 15, p. 30), therefore (as ? :
suming that the point of loading coincides with the point of attach
ment of the lowest conductor) the required height of pole to point
of loading = 20 + 4 = 24 feet approximately.
From Fig. 49, p. 96. we find that for a load of 710 Ib., at a
height of 24 feet, the diameter of pole at ground, level must not be
less than 102 inches.
If the arrangement shown in Fig. 26, page 45, is adopted, one ,
conductor will be 9 inches above the pole top and the other two
conductors 33 inches below, and the total height of the pole out of 
the ground should be 20 + 4 f 275 = 2675 feet. With 55 feet
SIMPLE WOOD SUPPORTING POLES
buried in the ground (see Fig. 55, page 110) the OVERALL Li
?OLE required = 2675 + 55 = 3225 feet.
From Table XII. , page 97, it will be seen that the
. tandard pole which realty meets the requirements is 34 J
inches, but a 32 feet/ 11 inch pole can be utilised if (which
.cacticable) the two lower conductors are raised 3 inches.
,ourse, leaves no margin for contingencies, but it is quite
if 250 feet is the maximum length of span. If 250 feet is tin
nngth some longer poles will clearly be necessary. In a
1 hen ordering it is advisable to obtain a proportion of the
; ngths next above (and a few below) that upon which tl
tions are based to allow for variations in length of span anc
. "ound. It may be noted in this connection that the safe
' ad of standard poles in the same series is approximately i
Mid independent of the length up to about 40 feet.
OF PRESSURE
FIG. 50.
This will be clear from the points plotted in Fig. 49, which
,;;w loading point 2 feet from the top of pole and the buriec
' a^shown in Fig. 55, page 110. Therefore, having decided i
writable length of pole for level ground it will not usually be
' y.wy to repeat the calculations for longer or shorter poles wh:
; 'io, doubt be required in some parts of the line (unless, of cou]
loading per foot run is increased in any way such as, for ex
*U double wired road crossings).
The use of standard sizes of wood pole is presumed throi
l!.us book, but it may often be possible to obtain nonstandar
nf economical prices.
Using 32 feet/ 11 inch poles the distances of the various
; '' 1(> ffi the ground will be as shown in Fig. 50.
Check The total BENDING MOMENT on pole at the groui
wiU be :
100 OVERHEAD POWER LINES
CAP WIRE JJ265 X 12) + 9/x 1775 = 58 000
ABM WIRES 24 x 12 x 355 = 102 200
EARTH WIRE (24 X 12) 28 x 1775 = 46 200
J 10 s ^ /
POLE ITSELF 265 x ^ X 8 x 1325 x 12 =. 28 100
Total B.M. in inch units 234 500
Therefore MINIMUM DIAMETER OF POLE required at ground
line for a Factor of Safety of 35
s /234500x 32 x 35 1A0 . ,
= V 7TX7800 = 10 ' 2 mclies '
The 32 feet pole selected has a diameter of 11 inches at a point
5 feet from the butt. The ground line is 55 feet from the butt,
therefore the diameter at ground line equals 11 ' X = 1094
* A.\J\J
inches approximately and the pole is therefore of ample strength.
We will assume in our calculations, however, that the diameter is
10*2 inches only.
Centre of Pressure due to Wind Load on Wires. Let
x = distance of centre of pressure from pole top (Fig. 50), then wo
have
1775(3 + 9) = 355(30  V) + 1775(30 + 28  x)
Whence x = 2725 inches. ^
Therefore the true height of pole to point of loading (i.e. the centre
of pressure)
= 265 ~?I5 = 2423 feet,
instead of 24 feet as assumed. This means that a diameter slightly
larger than 102 inches is necessary, but the difference is negligible.
Use of Chart to Determine the most Economical Span,
The stress in a conductor is inversely proportional to the nag and
if the proper sag is allowed, conductors of any size may be erected
on practicaUy any length of span with equal safety.
Prom a purely electrical point of view the longer the span the
better, since tiiere are then fewer insulators, which are the weakest
points in an installation. The modern tendency is to use long spans
resulting in a reduction rn the number of supports and in simplifying
the question of wayleaves. But it must not be forgotten that the
SIMPLE WOOD SUPPORTING POLES 101
individual size, weight and cost of the supports themselves and of
the insulators, arms, and brackets increases rapidly with span length
because :
(1) The sag is approximately proportional to the square of the
span length.
(2) The wind load is proportional to the span length.
It will be clear, therefore, that there is always a particular span
length for which, the product " Cost per Support X Number of Sup
ports " is a minimum.
If a chart is prepared showing
(a) Sag of wires at 122 F. in first quadrant,
(6) Wind load on wires in second quadrant,
(c) Safe nett loading of poles in third quadrant,
it becomes a simple matter to select the span length wliich gives
theoretically the minimum overall cost of the supports.
Fig. 51 has been prepared for this purpose for single circuit
high voltage lines with earth wire but without auxiliary conductors,
up to 400 feet span.
A similar chart will sometimes be found useful for L.V. lines.
For general use it is better to enlarge the chart considerably, and for
greater accuracy the second quadrant can be left blank until the
particular arrangement and number of conductors to be used is
known.
If compound poles are used Figs. 65, 66 and 67 on pages 132 and
133 are available for use in the third quadrant.
To illustrate the use of the chart Table XIII. has been prepared
for the 3/147 (05 sq. in.) H.V. distribution line in our example.
Where so manyiFacTfSfs are involved it was necessary to make a
number of simplifying assumptions, among which were : (1) Arrange
ment of conductors and earth wire as in Fig. 28 ; (2) vertical clear
ance between 'conductors on same side of pole, 1 foot for each 100
feet length of span ; (3) centre of wind loading on conductors at point
of attachment of lowest conductor.
In preparing the Table, values were tabulated for span lengths
varying from 150 to 400 feet at 10 feet intervals, but the only span
lengths selected for consideration are those which best fit the stand
ard sizes of poles. The cost figures given are naturally only ap
proximate and are for the supports only erected in fairly good
weather. They may have to be increased by 2030 % in average
English winter weather.
102
OVERHEAD POWER LINES
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SIMPLE WOOD SUPPORTING POLES
An average of Is. 6d. per annum per pole is taken for wayleaves.
A very usual charge for single poles is 2s. 6d. per pole on arable land
and Is. per pole on pasture. All other line costs, including con
140
s
90
t
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(19
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C&'N
ICE)
SOO 250 300 310
LENGTH OF SPAN (Fecr)
400
JFia. 52.. Cost of supports for high voltage overhead line. H.D. copper
conductors [05 sq. in.], Single wood poles.
ductors, stays, reinforcements at crossings, transport, supervision,
etc., are assumed to be independent of length of span. Bird guards
are not allowed for.
104 OVERHEAD POWER LINES
The cost per mile is plotted against length of span in Fig. 5
from which, it will be seen that the minimum cost per mile occn
with a span of 350 feet, but the variation in cost from 280 to 4(
feet is only about 5 %.
This result must be used with discretion. Assuming 131 fc
supports and 150 for conductors and earth wire it might be suj
posed at first sight that the line could be constructed for 300 or s
per mile. But a stretch of several miles of simple straight line ;
seldom practicable in this country and the reinforcements required a
angles, terminals and crossings add appreciably to the overall cos
of the line. The contract price for the line under consideratioi
would probably be about 500, to allow for overhead charges an<
profit, and therefore our saving of 5 % on the cost of the support
is reduced to less than 2 % of the overall cost of the line. It i
comforting to know, however, that unless we go to extremes 'the lengti
of the span has little effect upon 'the overall economy.
The cost of the supports under the 1923 E.G. Regulations ( in
ice) is also plotted in the figure and it will be seen that for the con
ditions assumed the reduction of the hypothetical ice loading tc
f inch has effected a saving of about 15 %.
In order to draw attention to a point which is sometimes missed,
the costs of supports for 193 conductors is also shown. It will be
noticed that the supports for the smaller conductor are the more
expensive. The reason is that, whereas the wind pressure on 3 / 147
is only 10 % greater than on 193, the latter has to be allowed 50 %
more sag than the former. The difference in this case is only 12
per mile, but in some cases it is larger and it may well be the deciding
factor when in doubt as to the advisability of installing a conductor
somewhat larger than absolutely necessary from purely electrical
considerations.
Further Notes on Selection of Span Length. It will be
noted in Table I., page 4, that on H.V. lines, the wind loading on
the largest conductor (7 / 193) is only about 1 times that on the
smallest (162), but that the sag required with the smallest conductor
is 4 times that required with the largest. Therefore, the larger the
conductor, the shorter the pole required for a given span length and
the longer the span possible for a given sag.
As a method of making a tentative choice of span length up to
350 feet, or so using standard single wood poles, it is suggested that
the length of span might be based on a sag of about 4 feet in still
SIMPLE WOOD SUPPORTING POLES 105
air at 62 F. for all sizes of conductor. This may not give the
theoretically most economical span, but all things considered; the
values so determined give good results.
Table XIV. 3 page 106, is given to illustrate the principles in
volved. In preparing the table, the span lengths to give 4 feet sag
at 62 F. were first selected from Fig. 17 and then altered slightly
where necessary so as to make the best use of the nearest size of
standard pole.
It will be noted that in some cases it is the length of pole which
is the limiting factor and in others it is the butt diameter. In the
former cases it will often be practicable to add a few feet by means of
ironwork so as to increase the conductor clearance from ground,
and in the latter it may be economical to use a pole longer than
necessary to get the requisite butt diameter and to cut off a few feet
from the top.
Foundations of Single Poles. It is laid down in the Elec
tricity Commissioners' Overhead Line Regulations that the supports
must be able to withstand the specified maximum hypothetical load
ing conditions without movement in the ground. This requirement is
reasonable with most forms of compound wood or iron structures,
since any appreciable movement may so alter the distribution of
stresses as to seriously weaken the structures. But in certain circum
stances a little movement may be most desirable in the case of simple
wood or tubular iron poles, particularly if failure of the pole itself is
thereby avoided. Moreover, the maximum ground reaction does not
become effective until the soil packs up a little due to a small move
ment of the pole.
Now, the properties of metals are well known, and we can pre
dict with reasonable accuracy the behaviour of good timber under
stress, but when we have to deal with soil we can only guess within
wide limits.
With regard to the foundations of simple poles it is established
that the pole tends to pivot about some point (Fig. 53 (a)) below
ground, level, but the exact location of this point is somewhat
uncertain.
If the pole is assumed to be absolutely rigid the horizontal dis
placement (d) at any point will be proportional to its distance from
this fulcrum [shown exaggerated in Fig. 53 (a}].
If it be further assumed that the soil has a definite elastic modulus,
which is inversely proportional to the depth, it can be shown that the
106
OVERHEAD POWER LINES
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SIMPLE WOOD SUPPORTING POLES
107
ground reaction stress diagram is parabolic in form, as shown in
. Fig. 53 (b), the fulcrum bein,
of resistance of the ground,
/ h
. Fig. 53 (6), the fulcrum being ~ from ground level and the moment
12
Jc.D.h*
12
. .
ID. ins.
D is the mean diameter of pole below ground level in inches, \
the depth in feet, f b the maximum rupture intensity of stress of the
soil in Ib. per square foot, k the maximum rupture intensity in Ib. per
square fool per foot of depth, and M Q the MOMENT OF RESISTANCE
oE ground in Ib. ins.
If, on the other hand, we neglect elasticity (which requires a
(ct)
PIG. 53.
stretch of the imagination when dealing with soil) and assume a
maximum rupture intensity of stress, which is directly proportional to
the depth, then the stress diagram is of straight line form, as shown
4
in Fig. 53 (c), the fulcrum ~= below ground and the MOMENT OF
Vfr
RESISTANCE of ground,
_ / . D . h 2 _Jc.D.h 3
10
10
Ib.ins.
It will be noted that for a given value of Jc, the value of h required
by the parabolic formula is only about 6 % greater than by the
straight line formula, and so for practical purposes the difference is
not very serious. It is generally thought that the parabolic formula
is more correct in the initial stages of the loading, but when the
108 OVERHEAD POWER LINES
foundations are on the point of giving way, the straight line formula
is probably more accurate, and it is therefore proposed to make use
of the latter formula in this book. The following experiment illus
trates its application.
A picket 5 feefc long, 3 inches in diameter, 3 feet in ground (loam)
began to give at 1 570 Ib. pull at ground level at right angles (Fig. 54).
3
in 10 X 1 570 X /TjX 12
.. Jfe = ^ = * = 4930.
D . h s 3 x 3 3
A similar calculation gives a value of lc, == 5 920 by the parabolic
formula.
In this case the picket was driven in, and the conditions were
rather more favourable than in the case of a pole fixed in a hole by
Fro. 54.
means of the earth thrown up in its excavation. If, however, the
ramming of the refilled earth is well done Je should eventually rise
again to the value it had in the virgin soil.
It will be appreciated that precise values cannot be given to
Jc for all the many different kinds of soil. Values up to 8 000 have
been obtained in good gravel soil, but for made ground it may be
less than 2 000. A conservative value for average good soil is 4 000.
With, regard to a Factor of Safety, the 1923 B.C. Regulations specified
25 for the foundations but did not suggest any figures for maximum
rupture intensity of the soil. The 1928 Regulations are more vague
on this point, but in the explanatory memorandum issued with the
Regulations it is implied that the foundations must be as strong as
the pole. It is proposed in this book to assume values for the
maximum rupture intensity of the soil and to allow a Factor of
SIMPLE WOOD SUPPORTING POLES 10<
Safety of 25 based on the specified weather loading and not on
the strength of the support itself.
It is believed that the foundation calculations in this and sub
sequent chapters will give sufficiently high factors of safety to satisfy
the Commissioners.
Buried Depth Required. Single Wood Poles. The
SAFE MOMENT OF RESISTANCE due to ground reaction, neglecting the
small difference between the diameter at ground level and the
mean diameter below ground, and allowing a Factor of Safety of 25,
D
10 X 25
Z) being the pole diameter at ground level in inches, h the depth in
feet and k the maximum rupture intensity in Ib. per square foot per
foot of depth.
The SAFE MOMENT OF RESISTANCE of the pole at ground level,
allowing a Factor of Safety of 35,
800
M G == *;: C" = 04& .D.h* Ib.ins.
Mp ~ 32 X 35
But the BENDING MOMENT referred to the fulcrum ~ feet below
ground level will be found to be some 10 to 20 % greater than this
value. Assuming the bending moment to be 15 % greater and
equating one to the other, we get
04/c . D . 7^ 3 = 116 X 219D 3 ,
,, 6300D 2
A ii  L
. . h _ ______
If k = 4 000 Ib. / sq. foot, Ji 3 157D 2 .
* For pole diameters from 7 inches to 13 inches the formula
h = 04D f 14 is quite near enough for practical purposes.
Curves connecting h and D are given in Fig. 55 for values of
k = 4 000 and 2 000.
From the lower curve we find that the pole selected in our example
should be buried 55 feet, and we will now check this value. The
BENDING MOMENT on the pole at maximum working load referred
{*(*
to a fulcrum ^ = 465 inches below ground level
= 234 500 + 465(1775 + 355 + 1775 + 1705)
= 234 500 + 41 200 = 275 700 Ib.ins.
110
OVERHEAD POWER LINES
Now the MOMENT OP RESISTANCE OF GEOUND when the pole is
buried 55 feet, assuming Jc = 4 000 Ib. / sq. foot
Jf. . JDJi 3 4 000 x 1039 x 66 3
= 691 500 Ib.ins.
10 10 X 12 3
.. FACTOR OF SAFETY against overturning
691 500
275 700
= 251.
J
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78 9 10 11 '12 13 H 15
Dia.. or Pole, at Ground Level in Inches
PIG. 55.' Buried depth of single wood poles.
The F. of S. is actually higher than this as the pole selected has a
much larger diameter than necessary.
If, however, the soil is poor, and we take a value of Ik = 2 000 Ib. /
sq. ft., the pole must be buried a depth of about 83 inches to be
selfsupporting with a F. of S. of 25. If buried 66 inches only, the
ground can only provide a moment of resistance of
SIMPLE WOOD SUPPORTING POLES
m
and therefore a moment of resistance of 275 700 X 25 345 750 ~
343 600 Ib.ins. must be provided by some form of fcmndatioii
reinforcement.
Assuming cross blocks of creosoted timber are used, one should be
placed at a distance of onethird the depth and the other at the full
depth, as shown in Fig. 56, as these positions are most effective in
the initial stages of the movement.
Let the blocks be 8 inches wide and the areas A,, (lower) and A &
3?ia. 56.
upper). If the stress diagram be drawn as in Eig. 56 it will be scon
hat the average pressures over the blocks are without serious error
2 000 66  4 , . . ,
= 72 Ib. / sq. m. on A
nd
t 12
2 000 2?
X ~ = 255 Ib. / sq. in. on A a .
If each block provides half the required reinforcement, then w
ave 3 taking moments about F,
72 X 155 x
255 X 245 X A z
Whence ^~^J_54 and A 2 == 275 sq. ins.
lowing for area of pole covered b^.iblocks, the total areas should bis
112
OVERHEAD POWER LINES
Suitable lengths would therefore be 2 feet 6 inches for A and 4
feet for A z , but conforming with the usual practice A 2 might be made
5 feet, i.e. twice as long as A : . A thickness of 4 inches would be
ample. (As a compromise, the pole may be buried about 76 inches
and the lower foundation block dispensed with. The amount of
excavation required will then be less.)
Size of Bolts for Cross Blocks. The loads taken by the
blocks a're approximately as follows (Fig. 56).
A z (Upper)
171
A l (Lower] ~
= TOGO IK
= 11 100 Ib.
Taking/^ for wrought iron as 65 000 Ib. / >sq. in., assuming 3 000
Ib. initial tension in bolt due to tightening up, and allowing a F. of
S. of 25, we have for
TT 737 7 D 7, 7
Upper Block Bolt d
r r?7 7 T> n 7
Lower Block Bolt d
10 OOP X 4
TT . 65 000
JTlOOx 4
TT . 65 000
A . '
= 44 in.
52 in.
d being the diameter at bottom of threads, it will be advisable
to use finch bolts in both cases. Two large washers should be used
with each of these bolts.
Shear Stress in Pole above Ground.It will be found that
the shear stress above ground level is quite negligible.
Below Ground. Load on Upper Block = 7 000 Ib.
Load represented by shaded area above the fulcrum F in Fig. 56
54
= z X 465 X 1022 = 12 800 Ib.
A
But these figures provide for a F. of S. of 25 in the foundation.
Therefore the maximum Shearing Force at F at maximum working
SIMPLE WOOD SUPPORTING POLES 113
the mean intensity. The diameter of the pole at F will be 1046 inch
approximately, therefore,
M r * v f co, o, 7 920 X 4 X 4
Maximum Intensity of Shear /Stress =  TTvJra  5
= 123 Ib. / sq. in.
Now the ultimate intensity of shear stress for red fir is about
4 000 Ib. / sq. in. across the grain and 500600 Ib. / sq. in. along
the grain. There is, therefore, nothing to fear across the grain,
but since the intensity of shear stress is equal on planes at right
angles there is a F. of S. of about 4 only along the grain at the point
F. However, the shear stress falls away from 123 Ib. / sq. in. at
F to 150 Ib. / sq. in. at the ground line.
Normally, the shear stress is not a limiting factor with single
poles but it is interesting to note that when single polos are tested
to destruction the fracture frequently shows signs of longitudinal
failure due to shear.
Shearing forces become of great importance in compound poles
of the "Butter " type (p. 116).
Deflection of Single Poles. It can be shown that, for loads
less than the elastic limit, the deflection of a cantilever of uniform
crosssection at the point of loading =
P . H 3 .
8 = ' inches.
3EJ
P  load in pounds, H the length to point of loading in inches,
E the modulus of elasticity in Ib. / sq. in and ,7 the moment of
inertia. Unfortunately E is not very accurately known for timber
but from experiments on fir poles it appears to have an average value
of about 12 X 10 6 Ib. / sq. in.
J for a circular section, about a diameter
= ~, and P for a F. of S. of 35 = 219 . ^,
219 X 64
X
_
3 . E . J ~ 3 x 12 X 10 6 "x^ F72T 4 '
00124 X ^inches.
8
114 OVERHEAD i POWER LINES
In our example, H = about 2423 feet and D = 100 inches,
therefore the deflection at the point of loading at the maximum
working load
94. Q2 y 1 92
= 8 = 00124 X 10 = 105 inc kes.
This is an elastic deflection, and the pole recovers when the.
loading is removed.
In the above the taper of the pole is neglected as is also wind
pressure on the pole itself, but owing to uncertainty as to the value
of E it is useless to pursue the matter further.
It is important to realise, however, that the poles are flexible,
and that the flexibility is an advantage. The lateral deflection"
introduces a component of the longitudinal tension in the wires to
help the pole laterally, and the longitudinal deflection which occurs
when one or more wires break results in a reduction in longitudinal
tension in the sound span and so tends to save the pole from
breaking.
115
CHAPTER VII.
COMPOUND WOOD POLES.
UNFORTUNATELY, for ordinary heights and loads, large portions of
trees have to be wasted. To reduce the waste to a minimum,
standard poles should be used, but it will generally be found un
economical to use single poles for lateral loadings much exceeding
1 000 pounds. For ordinary H.V. lines carrying 4 wires this means
a limit of span length of about 350 feet, but near the border line
there will be cases where it is financially sound to stick to single
poles, sacrificing a portion of the top in order to get a larger butt.
If auxiliary conductors are installed it will invariably be necessary
to use compound poles.
There' appears to be a general impression that the overall cost
of a transmission line is less if compound poles are used. This was
probably true in many cases before the revision of the E.C. Kegula
tions in 1923, but it is certainly not true today, at any rate for
single circuit 3phase lines. Compound poles require a good deal
more excavation work than single poles, and with the exception of
the " Butter " type they take up a good deal more ground space.
The only justification for compound poles in distribution work is the
reduction in the number of wayleaves required. This minimises
negotiation troubles, but there is no financial saving, since the rental
for compound poles is about double that for single poles.
Twin Poles. Consider two poles bolted together side by side,
as in Fig. 57.
T , 11 T , Am ^ 4 , ('K& d 2 \ 57rt 4
J VVl for each pole = / a a l + AS* = gj + (  X ^ J = ^.
y r 11 >7rtZ 4 1 57rt 3
' Z ^ for eadl p le = 6T X T = ir
116 OVERHEAD POWER LINES
K 3 *\ K 3 "\
and Z VVj _ for the twin pole = j X 2 = ^.
57rtZ 3
/.Z aai TT^ 1'
32
That is, the moment of resistance to bending in the plane of the
bolt of two poles arranged as in Eig. 57
is FIVE times the moment of resistance of
a single pole. This is interesting but
purely a theoretical result. In practice
it is not attained owing to the large
shearing forces which are called into
play and the bolts tear into the timber.
Rutter Poles (Fig. 58). This design
of compound pole makes use of the above
principle but provides for the shearing forces by a series of hard
wood scarf blocks set into accurately cut slots. In this way the
theoretical strength is realised ; in fact, as usually constructed with
4 inches between poles at ground line and 1 inch slots the Rutter
pole is about 8 times as strong as one of its members used singly.
That is the ratio of lateral to longitudinal strength is 4 to 1, which is
the limit imposed by the E.C. Regulations.
The shear blocks function as the web of a girder and are placed
closer together in the foundation portion, because in that portion the
shearing force is many times greater than in the part above ground
(see p. 150).
The resistance to overturning is provided by two long timber
foundation blocks, placed in the plane of the wires, and secured to
the poles by bolts. It will be clear that careful fitting is necessary,
and as the number of slots is large they should be cut before creosbt
ing. The construction is therefore essentially a factory job. This
type of pole is supplied by Messrs. Gabriel, Wade & English, Ltd.,
of Hull, and is delivered assembled, i.e. with the shear blocks and
bolts in position, which obviates reassembling on site.
The Rutter pole has a better appearance and is stronger, size "
for size, than any type yet designed.
^ A " Poles. This type of pole is still largely used in this
country for heavy lines. Let P (Eig. 59) be the'horizontal loading
COMPOUND WOOD POLES
117
34
Q
 3/iear Blacks jz "*6"*6"
~ Wes 9>te af Ground
M
O& Was/iera
 Shear B/oc/c 6"t6'*s'.3"
. Tbfi/bunc/at/'onB/oc/r
/i/// $/ee/>rr ro**S"i'S!a
Shear B/ocff <9'
eeper /a"xs"xf'6
oc/r /o"x6"*ef
FIG. 58. Butter pole.
at the apex, and and T the compressive and tensile loads on the
two members.
Resolving along and at right angles to the compression member
we have
118
OVERHEAD POWER LINES
O P ' j_
T sin oc = P cos.
2i
If, as is usual, the spread is equal to Jth of the height,
and
OC
~ = 3 approximately
cos
oc
2 = ^=8^ nearly
and
smoc
= T VP + 813P = 82P also.
That is, the compressive load on one member is equal to the tensile
load on the other, and about 8 times the horizontal
loading due to wind pressure.
For calculation purposes it is usual to assume
that, with the orthodox construction shown in Fig.
60, the compression member is equivalent to a strut
fixed at both ends for which Euler's formula
for the
BUCKLING LOAD = ' ' :
2 64. L* '
FIG. 59.
If J is taken for the mean diameter of the pole
and L the distance between the point of loading and the brace
block, we get a result which is roughly confirmed experimentally.
For example, consider an " A " pole made up of two standard
36 /11 in. poles
D m = 10 ins. and L = 388 ins.
and
64 . 388 2
= 155 000 Ib.
P = ^ = = 1890011 ,
O A O'Zi
(This omits the weight of wire and pole ironwork and of the pole
itself, which would add about 5 % to the compressive load.)
COMPOUND WOOD POLES
For a Single Pole
766 = 766 n ' 53
P A 18 900
119
' ' PS 3 470
G.I, Pole Roof
= 545.
I
4woshcr.
I'Dia: Tuba
Distance ple.ce.
irn JLL.,*.!
L \^3 0" H ;
FIG. 60 (a). FIG. 60 (6).
40 feet creosoted red fir " A " pole. 10 inches diameter at groundline.
6  75 inches diameter at top.
If these calculations are repeated for the 40 feet / 10 inch " A "
pole illustrated in Fig. 60 it will be found that P A = 7 750, P s =
2 030 and
P 7 7^0
. * A ' '" v/ Q.QO
' P7"2030~ B '
120 OVERHEAD POWER LINES
It cannot be pretended that there is anything very precise about
the above figures. No two poles are exactly alike and the timber
itself varies. The effect of the lower scarf bolt, the tie rod and the
earth reaction in reducing the free length of the compression member
has been neglected. Moreover, a good deal depends upon the fitting
which must be well done if the full benefit of the design is to be real
ised in practice. Poles constructed with a spread of oneeighth of
the length have been found experimentally (see paper by C. Wade,
JJ.JE.JE., August, 1907) to be approximately^ times as strong as
one of the members used singly, and in practice " A " poles are rated
on this value. The following instructions may be helpful in cases
where it is decided to construct " A " poles on the job.
" A " Pole Construction (see Eig. 60). Poles should be ap
proximately of the same dimensions and as straight as possible. If
not quite straight, 'curvature should be at right angles to " A "' plane.
1. Scarf the poles. Cut ends of! square. Arrange the poles
close together side by side with the tips supported on trestles.
Twist the poles into such a position that the proposed scarves will
be vertical and parallel to one another. Secure the poles tempor
arily in this position and draw parallel chords on the tips at
distances equal to one third of the tip diameter from the point
of contact. Then from the tip on the inside of the poles, mark off
a distance equal to six times the tip diameter and draw chalk lines
on each pole from these marks to both ends of the chords drawn
on the tips.
Remove the tapered portions with hand saw.
"When the scarved surfaces are laid together, the poles should
form an isosceles triangle with the butts at a distance apart approxi
mately equal to oneeighth of the height. If the distance apart
differs much from this value, the scarved surfaces must be planed
up until the correct ratio of height to base is obtained.
2. Fit the two scarf tie bolts. One bolt* to be 1 foot and the
other 3 to 4 feet (depending upon length of scarf) from tip. The poles
should be temporarily secured together at the tip with carpenter's
cramp and at the butt with rough timber and nails before commencing
to drill the bolt holes. Use  inch selfclearing auger. Make sure
that holes are horizontal and pass accurately through centres of
poles. 3 inch by 2 inch by  inch washers to be used.
The man using the anger can only check the direction in the
horizontal plane. Another man should look after the elevation.
COMPOUND WOOD POLES 13,1
3. Fix brace blocks. The brace blocks should be a driving fit
in the slots. The slots should not extend into the heart wood of the
poles. Brace block bolts to be 20 inches from butts.
4. Fit scarf block. Mark position of scarf block, which should
be of oak 6 inch by 3 inch section and of a length equal to the
diameter of the pole. The lower scarf bolt passes through the centre
of scarf block. A mortice 1 inch deep is first made on each side with
poles bolted together. Then separate the poles, finish slots with
saw and chisel and complete fitting the block.
5. Tar all cut surfaces. Give all mortices, slots and bolt holes
a coat of hot creosotetar mixture. Then rebolt up finally.
6. Fit roof. To be fitted transversely to the line of wires. Top
of pole beneath roof to be painted with creosotetar mixture.
7. Fix tie rod. Tie rod to be fixed at a distance from the butt
equal to about half the height of the poles out of the ground. Take
same precautions as in 2, but greater care is necessary in this case
as the holes in the two poles must lie in the same horizontal plane.
Distance piece of 1 inch G.I. pipe to be fitted over bolt between poles.
Four 3 inch by 2 inch by inch washers to be used.
It will be found with the construction described above., that
although the structure itself is 4 times as strong as a single pole,
if held rigidly at the base, the holding power of the ordinary type of
foundations, for the same depth, is not increased in anything like
the same ratio. To ensure having the strength of the foundations
comparable with the rest of the structure, it will be necessary to add
kicking blocks, and reinforce the connection between the poles and'
the brace blocks.
If the foundations give appreciably, the stress distribution in the
poles is considerably altered. This remark applies more particularly
to poles constructed with a splay less than oneeighth the height,
in which case the poles pull over and fail by simple bending.
See paper by Mr. W. B. Woodhouse, in J.I.E.E., February, 1929.
It is not the usual practice to attempt theoretical calculations
for the design of " A " poles, but to base the construction on a pro
cess of trial and error. It must be admitted that the factors involved
are so many, and the various conditions so uncertain, that calcula
tions unsupported by practical tests are of little use. Nevertheless,
an attempt will now be made to bring out the salient features affect
ing the strength of the scarf joint and the holding power of the
foundations.
OVERHEAD POWER LINES
" A " Pole. Scarf Joint. Consider the 40 ft. / 10 in. pol<
illustrated in Kg. 60 (a). The safe working load for this pole is
about 2 500 Ib. (Fig. 66, p. 132) applied 2 feet from the top, and the
direct load on each member with this horizontal load will be 2 50C
X 8 = 20 000 Ib. approximately. Actually, of course, there will be
a number of wires at various distances from the top, and the centre
of pressure will generally be more than 2 feet from the top. Bui
from the nature of the problem close accuracy cannot be claimed foi
the calculations.
First assume two fincli bolts to be used alone. The bearing
lengths of the upper and lower bolts are about 5 inches and 7 inches
respectively in each pole.
. CO (d).
First consider the tipper bolt. If there were no flexure the joint
would fail through crushing of the timber, the stress in which is
roughly indicated by the shaded triangles in Fig. 60 (c).
In, practice, however, the moment of resistance of the timber
on the outside halves of the bolt assisted by the frictional force
between washers and poles is greater than the moment of resistance
of the bolt, and therefore when the joint fails the bolts bend from
their centres, as shown exaggerated in Fig. 60 (d). Neglecting the
small inclination with the vertical, if P = longitudinal load on each
pole and/ c the maximum safe compressive stress in timber in the
bolt hole (1 030 Ib. / sq. in., vide p. 77), each of the triangular loads
X
965 
Ib.
COMPOUND WOOD POLES
== = direct uniformly distributed compressive load, not shown
5 X *7o
in diagram).
The C.G. of this load is f X f = inches from centre of bolt,
therefore its moment of resistance
/ p\ 5
= {1030r]Slb.ina.
995 Ib.ins.
The moment of resistance of the bolt
nD* f TT . 27 . 60 000
32 J ' 32 X 64 X 25
therefore the total MOMENT OP KESISTANCE
= J2 X  x (l 030  ) f 995 lb.ins.
The BENDING MOMENT = 5P ; equating one to the other we get
582P = 4 430 and P = 762 Ib.
Working similarly the safe load on the lower bolt will be found to
be 895 Ib., making a total of
762 h 895 = 1 657 Ib.
The absurdity of such a
joint will be at once apparent,
so we will proceed to consider
the state of affairs when a 6
inch by 3 inch oak block is
inserted in the scarf at the
lower bolt.
The diameter of pole where
block is inserted will be about
7 inches.
e 2
FIG. 60 (&
571, .. d = 110 (Fig. 60 (e)).
a
Length of Chord, AB = 2 X 35 X sin  = 7 X 82 = 574 inches.
Length of Arc, AGB = 7rXJ>X^ = 7rX 7^^ = 673 inches.
fi
Area of Sector, OACB = TT X R z X
360
110
= TT X 35 2 X ^ = H8 sq. ins.
OVERHEAD POWER LINES
5*74
Area of Triangle, OAB = 2 X ^ = 574 sq. ins.
... Area of Slot = 118  574 = 606 sq. ins.
Distance of centre of area of sector from centre
___ 2 Eadius X length of chord
= 3 X length of arc '
2 X 35 X 574 , nn . ,
 3 X 673 = X '" mCll6S 
2
Centre of triangular area = 2 X 5 == 1*33 inches from 0.
o
If centre of slot area is x inches from 0,
we have QOQx + 574 x 133 = 118 X 199,
whence x = 262 inches.
Now, if the scarf block be assumed to take the whole of the load,
the average intensity of pressure on the timber in the slot will be
20 000 OAA ,, . . . . TJ, , f o , , f 6 000
 = 3 300 Ib. / sq. in., giving a JB actor of Safety of ^^.
D'Oo o oOO
= 182, which is insufficient. The maximum permissible load on
the scarf block  X 606 = 10 400 Ib.
oo
Required Minimum Depth of Scarf Block. We will
assume that the load of 10 400 Ib. is uniformly distributed over the
area of the slot.
As the centre of area of the slot is 262 inches from 0, it must
be 062 inch from AB, therefore : ,,y,
the BENDING MOMENT on the block =, 10 400 X 62 = 12 900 Ib.ins.
The horizontal reaction will be a maximum at the upper and lower
edges, falling away to zero at the centre, i.e. the load diagram will
be triangular in shape, as shown in Pig. 60 (b), page 119.
The total permissible load on each side of the centre, if d is the
depth of the block,
JL 0\J\J JL ^j Cv rftf^iii
= ^g X ^ X 574 X g = 615cZ Ib.
o
.. MOMENT OF RESISTANCE = 6I5d x ^d = 410d 2 .
o
Equating the Bending moment to the Moment of resistance we get
410cZ 2 = 12 900,
and d = 561 inches.
COMPOUND WOOD POLES
Frictional Forces. These may be estimated as follows :
The tensile load on the lower scarf bolt will be about 3 000 lb.,
due to screwing up and on the upper bolt 3 000 lb. + 615$ =
3450 Ibs. due to the scarf block reaction. Assuming a coefficient
of friction of 03 for wood on wood, the vertical reaction will be about
9 450 X 3 = 2 835 lb. This is somewhat indefinite, perhaps, but
quite appreciable.
Help Given by CrossArms. Assume a 4 inch by 2 inch
channel section crossarm to be fitted into 1^inch slots as shown
dotted in Fig. 60 (6), page 119. It will be clear that any tendency
of the compression member to ride upwards on the tension member,
due to failure of the scarf joint, will twist the arm counterclockwise.
The stress diagram in the timber at top and bottom of slots will be
triangular, as shown.
Assuming pole diameter to be 7 inches, the modulus of the slot
section will be
Z c = ~bd z = 4 X 15 x 287 2 = 329 inch units
15 15
(see p. 80). If the arm bolts are 9 inches apart, and P = the
maximum permissible direct load on the slots, we have roughly
9P=2/ C Z C ,
D 2 X 6 000 X 329
' P = 9X35 =
In addition to this Moment of Resistance of the slots there is a
clockwise Bending Moment due to the wind pressure on the con
ductors, if pin insulators are used (see p. 78).
Assuming 600 lb. wind pressure on each of two conductors, and
that the conductors are attached to points on the insulators 8 inches
from centre of crossarm, the bending moment due to this eccentric
loading on the crossarm = 600 X 8 X 2 == 9 600 Ib.iiis/ and the
vertical load on the slots = = 1 067 lb., of which 5= == 305
y o'D ,
lb. may be added to the vertical working load.
Similarly, if a second arm is fitted higher up where the distance
9
between bolts is 6 inches, further additional loads of 1 250 X TT
6
9
= 1 875 lb., and 305 X r = 457 lb. can be dealt with.
126 OVERHEAD POWER LINES
With, two arms and a scarf block, therefore, the TOTAL LOAD whic
the pole top joint will support
= 10 400 + 1 250 + 305 + 1 875 + 457 + 762 = 15 049 Ib.
(7G2 Ib. is allowed for the upper scarf bolt, but the lower one i
neglected, since, in addition to the tensile stress due to tightenin
up, it has to withstand the reaction between the scarf block and th
two members.)
This is still a good deal less than 20 000 Ib., the value based upo]
the assumption that the pole is 4 times as strong as one of it
members used singly, and to enable the full strength of the pole t<
be developed a second scarf bloclc is really necessary if, as assumed
the load is applied 2 feet from the top.
In practice, however, as the centre of pressure would be mucl
greater than 2 feet from the top, and friction has been neglected
there is no doubt that the Factor of Safety would be adequate witl
a load of 2 500 Ib.
If a second scarf block is fitted it will be necessary to check tin
shearing strength 6f the pole above the block.
"A" Pole Foundations. In practice the foundations ar<
found to fail by the pulling out of the tension member. It is there
fore proposed to assume that the brace block bolt in the com
pression member is a fixed point.
The figures for ground stress will be taken from Fig. 91, page 181
The upward ground reaction will be assumed to be equal to tli<
horizontal ground reaction at the same depth. It is actually greater
but it is important to avoid any settlement. The horizontal loac
of 2 500 Ib. applied at a point 2 feet from the top of the pole pro
duces a BINDING MOMENT of 2 500 x 8 x 60 = 1 200 000 Ib.ins
on the foundations, if we neglect the small inclination of the pole
with the vertical. The upper surfaces of the brace blocks are 5
inches from ground level, at which depth the safe upward bearing
pressure = 105 Ib. / sq. in. approx.
The downward ground reaction will be triangular in shape, as
shown in Fig. 61, and the total safe load jr X 84 X 10 = 4 400 Ib.
2i
The C.GL of this load = 84 X f = 56 ins. from the brace block
bolt, therefore the MOMENT or RESISTANCE = 4 400 x 56 =
246 000 Ib.ins.
To this may be added the MOMENT OF RESISTANCE of the com
COMPOUND WOOD POLES
127
pression member to lateral earth pressure. Since there is a large
volume of disturbed soil and any appreciable movement is undesir
able, we will take k = 2 000 in the straight line formula given on
page 107.
2 000 X 105 X 78 3
Then
M
10 X 12 3 X 25
= 231 000 Ib.ins.
,78
This implies that the compression member pivots about a point
= = 55 inches below ground level instead of 58 inches as assumed
otherwise in these calculations, but this makes no material dif
ference to the result.
78"
49"
J.
Mv
/Two Brace Blocks.
I0"5"i<9'lot1<j.
Kicking Blocks
1 "''U'O'elon^.
/ i
D
H 3' 6"
FIG. 61.
p, ,, We may also add the MOMENT OP RESISTANCE of the slots which,
estimated as explained on page 79, will be about 43 000 Ib.ins.
ft There may further be a little help given by the upward reaction
<Jn the brace blocks outside the compression member, but this will
tjiie very small owing to the fact that the brace blocks share the large
Downward load of 20 000 lb., which produces a crushing stress on the
;oil exceeding 30 lb. / sq. in. The maximum safe downward stress
tjg ,!;3ijp. the soil at a depth of 63 inches, assumed equal to the safe horizon
:y' };al stress, is about 45 lb. / sq. in.
'"' The total MOMENT OP RESISTANCE, then, which can be counted
upon = 246 000 + 231 000 + 43 000 = 520 000 Ib.ins., which is
, quite inadequate.
It may be noted here, that, prior to 1923 the B.C. F. of S. for
128 OVERHEAD POWER LINES
wood poles was 10, and the maximum bending moment on these
35
foundations would have been 1 200 000 X ^r  = 420 000 Ib.ins. only,
to deal with which a simple single brace block would have been
considered quite sufficient ; and in fact proved to be so in practice.
It must be pointed out, however, that after a year or so, when the
earth gets consolidated, the strength of the foundations is greatly
increased and may in favourable circumstances reach double the
initial value when the pole is erected.
Now that the F. of S. for wood poles has been reduced to 35,
it is obvious that more attention must be paid to the foundations,
and we will now consider the effect of adding KICKING BLOCKS at
the ends of the brace blocks.
The detailed calculations giving the moment of resistance due
to the upward ground reaction on the brace blocks and kicking blocks
at the foot of the compression member will be left as an exercise for
the reader. Together with the moment of resistance of the slots it
will be found to be nearly equal to the MOMENT OF RESISTANCE of
the brace blocks themselves which
= Hf = 2X5X100X7800 = ^
6 6x35
The moment of resistance to be provided by the kicking blocks at
foot of tension member is therefore
1 200 000  246 000 372 000  231 000 = 351 000 Ib.ins.
If the kicking blocks are 4 inches thick their upper surfaces will
be 49 inches from ground level. The safe upward ground stress
at this depth is about 9 Ib. / sq. in., therefore the area of the kicking
blocks must not be less than . ,
351000
Tf 1 2 innhp.s wide, a length of 3 feet 6 inches will do.
Load on Slots. Working as shown on page 123
; the area of the 2inch slot is about 154 sq. ins.,
iner edge 866 inches, and the distance of C.G. of
COMPOUND WOOD POLES 129
slot area from inner edge 102 inches. Neglecting the area of the
bolt itself, the compressive stress on timber in slots
20000 .
= 15^2 = 650 lb./s q . m.
[neglecting the relief due to lateral ground reaction].
The safe working stress on the timber in compression
6000 , _ K1 , .
_  = 1 715 Ib. / sq. in.
O'O
and from this point of view, therefore, the slots need not be so
deep. About 1g inches would be sufficient.
Shear Stress. The stress tending to shear off the pole below
the slots
650 X 154
The maximum safe longitudinal shear stress
500 _ . Q .
= = 143 Ib. / sq. in.
o'D
There is ample security here also, and we could safely place the
brace blocks several inches nearer to the butts.
Twisting Moment on Brace Blocks. The above cal
culations for stresses in compression and shear are only true if
there is no appreciable movement of the brace block in the slots.
If we assume uniform stress distribution over the bottom of the
slot, the C.G. of the slot reaction will be 102 inches from the inner
edge, and taking the C.G. of the load on the brace block to be on its
centre line there will be a TWISTING MOMENT on it = 10 000 (25
102) 14 800 Ib.ins. (Fig. 62 (a)). Now the slot fixing is quite
incapable of dealing with. this. If the brace block twists ap
preciably the concentration of stress near the outer edge of the
bottom of the slot will be so great as to crush the timber, the brace
block will become loose and as the f inch bolt has a safe moment
of resistance of 995 Ib.ins. only the joint will fail.
It may be noted that the centre of the brace block' is not the
best position for the bolt in the tension member. It will be much
more effective if placed well above the centre (say 2 inches above),
but in this case a second bolt should be placed below the centre to
satisfy the conditions when, owing to change of direction of wind,
9
130
OVERHEAD POWER LINES
the member becomes loaded in compression. Moreover, if a narrower
brace block is used and housed full depth, the twisting moment will
be very much reduced, but reducing the size of the brace blocks will
necessitate further modifications in the design. Fortunately, in the
case under consideration, a considerable moment of resistance is
provided by the fact that the kicking blocks are bolted to the brace
blocks.
The compressive load between brace blocks and kicking blocks
is indicated by the small triangles in Pig. 62 (6), and the safe maximum
MOMENT OF RESISTANCE
1500 5
5
= 6 430 X 167 X 2 = 21 400 Ib.ins.,
which is greater than necessary.
10,000 pM
I4&"
I r
rf"
f
T ~
jf
4"
ik!
t.
\
10"
J
j,
5.
i
i
\
i
i
i
i
i
U"
1
1
^
F
i
i
4"
HI
~ i ...
PIG. 62 (re) in slota. FIG. 62 (6) at kicking blocks.
Brace block sections.
The stress in the finch bolts securing the kicking blocks to the
brace blocks allowing 3 000 Ib, tension due to tightening up the
nut
3 000 + 6 430 X U 8
21400
304
= 24 500 Ib./ s<j. in.
The maximum safe working stress = 26 000 Ib. / sq. in.
a'O
It is not proposed to pursue this matter further as it is thought
that enough has been said to indicate the lines on which the strength
of the foundations may be estimated.
COMPOUND WOOD POLES
131
Wo have neglected the loss of area of brace blocks in slots and
under kicking blocks and a number of other small details, but the
method of calculations will bo found to give results which agree
closely with those obtained by experiment on poles shortly after
erection in reasonably good soil. Better results will generally bo
obtained when, the soil gets thoroughly consolidated (say after six
to twelve months).
If __J L xf. J
 J>l n fy Q " f]
I'ru. 63. Butter fcyyo.
METAL STRAPS,
.Fxu. 04,Aiu!hoi.'a typo.
H of Hfcrongtihonod "A" jiolo fuuudal/itniH.
A, good deal of tlvought has been given during the last two years
to the design of satisfactory "A" polo foundations and the two
designs illawtrated in Figs. 03 and 04 aro of interest.
Rutter " A " Pole Foundation. Fig. 03, shows a new type
of " A " pole foundation patented by Mr. Kutter. Braces are housed
full depth in the poles, near the bottom, and the usual transverse
kicking blocks aro provided. The braces are connected to the poles
132
OVERHEAD POWER LINES
8000
Wind Load on /ce Covered Wires in Lbs.
7000 6000 5000 4000 3000 2000 1000
\\
\
\
\
V\
X
XA
\u
r\
\
10
Diameter of Pole at Ground Line in Inches
Fio. 65. Net safe working loads. Rutter poles.
Wind Load on Ice Covered Wires in Lbs.
7000 6000 5000 WOO 3000 2000 1000
Diameter of Pole at Ground Line in Inches
IG. 66. Net safe working loads. "A" poles.
COMPOUND WOOD POLKS
133
Ly stirrup straps which are proportioned to the load both in sectional
area and length, so that a sufficient number of bolts can be provided.
Using narrower brace blocks housed full depth reduces the bending
moment on the bolts, and incidentally the stirrup straps enable the
brace blocks to be fixed nearer the butts.
Anchora "A" Pole Foundation. Fig. 64 illustrates this
type of " A " pole foundations, which has recently been patented.
The special features of the design are (1 ) the wrought iron tie rods
which transfer the upward pull from the kicking blocks to the
tension member ; (2) the undercutting of the earth to take the kicking
4000
3500
Wind Lo&d on Ice Covered Wires in Lbs,
3000 2500 2000 1500 1000
Diameter of Pole at Ground Line in Inches
Kid. 07, Nob Hiife working loads. "PI "polos.
blocks, thus taking a proportion of the \ipward load by undisturbed
soil ; (3) the brace blocks are situated in their most effective position,
viz. at the pole butts (maximum depth) ; (4) the metal plates of
channel .section fitted to improve the bearing surface under the
butts.
" H " Poles (Fig. 85, page 174). "H " poles, as usually con
structed with one set of trussing tackle, are not so strong laterally
as " A " polos, but they arc simpler to construct.
They are "largely employed, for terminations, junction and sec
tioning poles and for road and telegraph crossings where a greater
ground clearance is required, owing to the longer arms which are
134 OVERHEAD POWER LINES
possible and the extra space available for dealing with, conductors
stay and guard wires and for fixing pole type transformers, isolating
switches, cable terminating boxes, etc. Also it will frequently b<
necessary to use them at angles and terminals in cases where th<
direct compressive load is too much for a single pole.
Eigs. 65, 66, and 67 give for BUTTER " A " and " H " poles
respectively, the safe load that may be applied to a point 2 feel
from the top of a pole, taking a modulus of rupture of 7 800 Ib. pei
square inch and deducting the wind pressure calculated at 8 Ib. pe7
square foot, after dividing by a Factor of Safety of 3 as required
by the E.C. Regulations.
For the " A " and Rutter poles, the wind pressure is taken on
1 times the projected area of one pole, and for the " H " pole If
times.
The figures for RUTTER poles are obtained by calculation, allow
ing for 'the weakening due to the 1inch slots and one finch bolt.
" A " poles are assumed to be 4 times and " H " poles 3 times
as strong as one of the members used singly.
It is understood that these assumptions are 1 approved by the
Electricity Commissioners.
135
CHAPTER VIII.
, IRON AND FERROCONGRETE POLES.
IRON POLES are not often used if wood poles are suitable and easily
procurable. Por the same duty the former are much more expen
sive than the latter, and as iron poles have to be painted periodically
they cost more in maintenance.
In the colonies and India, however, climate and the white ant
frequently make the use of iron poles imperative. In this country
TUBULAR IRON poles are employed occasionally in residential dis
tricts for aesthetic reasons since they take up less room and lend
themselves better to painting and decoration.
The British Engineering Standards Association have recently
issued a specification for TUBULAR IRON AND STEEL POLES for
Telegraph and Telephone Purposes (B.S.S., 134r 1927) from which
the following particulars have been abstracted by permission of the
Association. Official copies of this specification may be obtained
from the offices of the Association, 28 Victoria Street, London, S.W. 1,
price 10s. 10d. 3 post free. A good many of the standard sizes are
suitable for power distribution, including 37 feet poles for working
loads up to 1 000 Ib.
Four types are standardised, two giving lengths up to 44 feet
in two or three parts, and the other two made up of short lengths
not exceeding about 8 feet each. These latter " multiple section "
poles are the most suitable type for use abroad, as their handling,
storage and transport are simpler.
" Type B " consists of a series of tapered steel riveted tubes,
galvanised after manufacture, with a cast iron base so combined as
to give the required strength and length. Any series of tubes car
be nested together into a package 8 feet long, the diameter of whicl
is equal to that of the largest tube. This method of packing avoids
the necessity for any further packing and ensures economy in ship
ment.
136
OVERHEAD POWER LINES
To construct tlie pole, tlie bases and tubes are laid on the groun
and the tubes so placed that the riveted seams appear on alternai
36.4
S'.B
(a)
PIG. 68. Riveted seam tapered pole in multiple sections.
British standard Type B.
Test load 1 100 pounds applied 6 ins. from top.
sides. The lower tube is first driven on the base, and the seconc
tube driven on to the lower one, and so on, until the pole is complete
As an example, Fig. 68 (a) shows the assembly of a Type B stan
IRON AND FERROCONCRETE POLES 137
clard pole of overall length, 36 feet 4 inches (nominal 35 feet) capable
of withstanding a test load of 1 100 Ib. applied at a point 6 inches
from the top, when buried in the ground 5 feet 6 inches. Fig. 68 (b)
shows the joint between tubes F and E, Fig. 68 (c) the tube F, Fig.
68 (d) the cast iron base, and Fig. 68 (e) the buckled plate. The
ultimate tensile strength of the steel is specified as 2732 tons per
square inch, and the elongation 20 % on an 8inch test piece. The
ultimate tensile stress of the C.I. used in the base must not be less
than 9 tons / sq. in.
The yield point stress in the steel tubes, calculated from specified
cantilever test loads must not be less than 20 tons / sq. in., and the
C.I. Base must withstand a test load producing a calculated stress
of 9 tons / sq. in. without breaking.
The test load for the assembled pole is based on the assumption
that the ground does not give effective support for a distance (in the
case of this particular size of pole) of 10 inches. For purposes of
calculation, therefore, the " length above the ground " is taken as
equal to 30 feet 10 inches + 10 inches = 31 feet 8 inches. The
bending moment on the section 10 inches below ground level is
therefore 1 100 X (380 6) = 411 400 Ib.ins. when the test load
is applied.
As a matter of interest we will check the stress at two of the
sections.
Maximum Stress in Steel Tube F, just below top joint (at
fitting line). From Fig. 68 (a) it will be seen that, at the section
chosen, the approximate external and internal diameters of the tube
are 90 and 8744 inches respectively and the distance of the section
from the point of application of the load = 261 inches. '
D 4 d*
The MOMENT OF RESISTANCE of section = TT / m which
D and d are respectively the external and internal diameters and /
the maximum stress.
The BENDING MOMENT = 1 100 X 261 Ib.ins. ; equating one
to the other we get
f _ *r(9*8.7M')
J ~ J
32D ~ 32 X 9
whence /= 37 000 Ib. / sq. in. = 165 tons / sq. in., which is well
within the specified figure of 20 tons / sq. in.
138 OVERHEAD POWER LINES
Stress in Cast Iron Base 10 inch below ground level
Beference to Fig. 68 (d) will show that the cast iron base has, a
the point chosen, the approximate external and internal diametei
of 8325 and 7325 inches respectively, and the distance of th
section from the point of application of the test load = 374 inches
7r(8325 4 7325 4 ) f , 1AA ot _. n .
... 32 x 8 . 325 ^ 1 100 X 374 lb.ms. 5
whence / = 18100 Ib. /sq. in. = 81 tons/sq. in., which is we
within the figure of 9 tons / sq. in. which the material has to with
stand to comply with the specification.
Assuming an ultimate stress of 30 tons per square inch, the maxi
mum working load for this pole will be $ 660 Ib. applie<
.Z'O a\J
380
6 inches from the top or 600 X ^^ = 694 Ib. applied 2 feet from th
top.
Allowing for wind pressure on the pole itself, it will carry a H.V
line (f inch ice) consisting of three 05 s'q. in. copper conductors an<
one 7/08 earth wire with the centre of pressure 2 feet from top on i
span length of 220 feet, but the wires will be nearly 6 feet highe
than they need be. If the conductors are lowered 4 feet, the spai
length may be 250 feet, and of course the top member " c " need no
be more than 4 feet long.
In tropical countries the pole should be suitable for longer spans
The total weight of the pole is about 800 Ib. (tube C, 65 ; D, 92
E, 120 ; F, 154 ; cast iron base, 350 Ib. ; and the buckled plate 1*
Ib.).
The weight of an equivalent wood pole would be about 900 Ib
If weight is an important consideration, it may be noted tha
Type D poles for the same duty can be obtained in high tensil<
steel (up to 40 tons / sq. inch), weighing 520 Ib. total, the maximun
weight of any one part being 204 Ib. But since the steel tubes ar<
buried in the ground, and no buckled plate is used, such poles shoulc
be set in concrete.
Foundations (see p. 105).
/
BENDING MOMENT referred to apoint = = 467 inches belov
V2
ground level = 660(364 f 467) 271 000 lb. : ins (Fig. 69).
IRON AND FERROCONCRETE POLES
D . Jc . Ji s
MOMENT OP RESISTANCE OF GROUND = M =
139
(p. 107).
It will be seen from Fig. 68 (d) that the average value of D below
ground level is 86 inches. Assuming then that Jc 4 000,
86 X 4000 x 66 3
10 X 12 3
.. F. of S. against overturning
= 571 000 Ib.ins.
571 000
271 000
= 211.
660 LBS
FIG. 69.
This is insufficient, but it is usual, however, to add a buckle plate,
20 inches square fox the pole we are considering.
From Fig. 91, page 181, we find that the maximum pressures
which the ground will withstand at a depth of 5 feet 6 inches are :
(1) Uplift at end A (Fig. 70) = 2 360 X 25 = 5 900 Ib. / sq. ft.
(2) Downward at f
end 5=7 080 X 2.5
= 17 700 Ib. / sq. ft.
This assumes the
downward pressure
which the ground can
support to be equal to
the pressure offered to
a stay block when
pulled horizontally.
Actually the former
should be much greater than the latter.
To these figures may be added the weight of the pole, wire, ar
fittings, which will be about 1 000 Ib., giving a downward pressure
X 144 = 360 Ib. / sq. ft. Therefore the maximum pr<
.20 X ^0
sures when the ground is on the point of failure will be :
13Slbs
Fia. 70.
140 OVERHEAD POWER LINES
(1) End A, 5 900 + 360 = 6 260 Ib. / sq. ft. = 435 Ib. / sq. in.
(2) End B, 17 700  360 = 17 340 Ib. / sq. ft. = 120 Ib. / sq. in.
The load diagrams on the upper and lower surfaces of the buckle
plate will be as shown in Fig. 70, therefore the moment of resistance
due to the plate
= 109 000 Ib.ins.
This neglects the loss of area on the upper side of the plate due
to the. pole, but this introduces no appreciable error. Total moment
of resistance of ground will then be 571 000 + 109 000 = 680 000
Ib.ins., and the F. of S. against overturning is increased to
680 000 r . , .
271 OQO = 2 ' 5 approximately.
If the buried depth is increased from 66 to 70 inches the F. of S.
goes up to about 30.
Deflection. It is not possible to calculate this very closely,
but we can make a guess at what to expect, in the following way :
PH 3
In the case of a uniform tube the deflection 8 = ^^^ and
3EJ
if the tube were conical with the apex at the point of loading,
JD//3
= . The actual deflection will be somewhere between these
2EJ
two values.
Assuming the deflection to commence at the top of the lower
band in tube F, then H = 3.25 ..inches, external diameter of tube =
10026 inches and internal diameter =977 inches approximately.
T
* / r _________ !
7r(10026 4 977 4 ) ,
" i " ........... . ...... ..... *"..'.'"" 4ri
64 64
Taking E = 30 . 10 6 Ib. / sq. in.,.
66QX325 3
_
~Wj~ 3 X 30 X 10 X 48 
For a conical tube the deflection equals 157 X 15 = 235 inches.
So a rough estimate of the deflection to be expected at the maximum
working load is the mean of these two values, say, 19 inches.
Channel Iron Poles. The tubular poles described above
would naturally be used if available, but in some cases abroad it
IRON AND FERROCONCRETE POLES
141
may be necessary to make up poles from sucli standard sections as
may happen to be at hand. Channel Section is very suitable for
the purpose.
(a) (6)
If
ftr i
fca^
DETAIL. / J
2."xs"xfa
AT'AV g
s*
Washer
(t
/ 4
jfe^^j
? '
n
6"
% SOLT5
Jfia. 71. Simple channel iron pole.
Fig. 71 (a) shows a suggested design for a channel iron pole which
is approximately equivalent in strength laterally to a 32 foot by
9 inch (i.e. medium) fir pole.
The pole is made up of three lengths of standard channel, one
11 feet of 6 inches by 3 inches, one 11 feet of 5 inches by 2 inches,
142 OVERHEAD POWER LINES
and the other 12 feet of 4 inches by 2 inches. The lengths overlap
one foot and are bolted together.
A simple way of studying the strength of the pole is to draw a
MOMENT OF RESISTANCE diagram (Fig. 71 (&)) from the figures given
in Table VIII., page 71. It will be seen from this diagram that,
assuming 5 feet G inches buried, and that the point of loading is
2 feet from the top, the pole is suitable for a BENDING MOMENT of
about 147 000 Ib.ins., with a F. of S. of 25. Allowing for wind
pressure on pole itself, the lateral load which the pole will carry
14 7000
245 X 12
21 = 500 21 = 479 Ib.
The pole is therefore suitable for a H.V. line (finch ice) with
three 05 sq. in. conductors and a 7 / 08 earth wire on a span length
of 175 feet.
The total weight of the pole is about 440 Ib. and that of an equiva
lent wood pole 600 Ib.
Ratio of Lateral to Longitudinal Strength. (See Table
VIII., p. 71.)
709
6 ins. X 3 ins. Channel Section == ^ Q = 53 [457 only utilised], ,
.L'iJO >)
4749
5 ins. x 2 ins. ,, = ^g = 50,
4 ins. X 2 ins. ,, = ^^ 505 [44 only utilised]. J
The B.C. Regulations specify a maximum ratio of 4, therefore in
the unlikely event of such a pole .being used in this country, the
loading would have to be reduced from 500 Ib. to 500 X & 400 Ib.
It will be noted from Fig. 71 (b) that the strength of the pole is
determined by the strength of the 5 inch X 2 inch section.
Strength of Foundations.
Bending moment referred to fulcrum (Fig. 71 (a)) p. feet
v 2
below ground level
= 500 (245 X 12
\
= 147 000 + 23 300 = 170 300 Ib.ins.
IRON AND FERROCONCRETE POLES 143
MOMENT OF RESISTANCE of ground (p. 107).
Dkh3 3 x 4 OOP X 663
10 x 12 a
.
200 000 lb,ms.
.. F. of S. against overturning = 1 . = 117, which is insiifficient.
X i o\j(j
If a sole plate is attached to the foot of the pole, it will help
matters, but the calculations on p. 139 show that we cannot hope
for more than another 100 000 Ib.ins. from a substantial 20 inch
by 20 inch sole plate. Cross blocks will therefore be necessary, and
these should preferably be of concrete slabs (seep. Ill for calculations).
However, it is perhaps better to set such poles in concrete. The
stability can then be easily ensured and, moreover, steel embedded
in concrete lasts indefinitely.
A concrete block 12 inches X 12 inches X 72 inches is suggested.
The bearing surface width will then be increased to 12 inches and
the depth to 72 inches.
Now, although a slight movement in the ground is unobjection
able when simple wood and iron poles are planted direct in the ground,
it is advisable to ensure as far as is reasonably possible that there
shall be no movement whatever of a concrete foundation. This is
of greater importance in the case of lattice girder structures than in
the simple pole under consideration, but it is recommended that in
all cases when calculating the moment of resistance of the lateral
earth reaction to concrete pole foundations the value of k should be
taken as 2 000 in good soil.
The MOMENT OF RESISTANCE will then be
12 . 2 OOP . 72 3 MQAnnl1 .
= _ =  1Q 123  = 518 000 Ib.ins.
and the F. of S. against overturning = 30.
This neglects the small moment of resistance due to unsym
metrical earth pressure under the block.
Concrete Pole Foundations. The concrete should be com
posed of clean gravel (or ballast) or hard broken brick or stone,
with sharp clean sand mixed with Portland cement in sufficient
144 OVERHEAD POWER LINES
proportions to fill the interstices of the coarse material. The
mixtures usually employed are :
1 cement, 5 sand, 10 gravel or broken stone (graduated) ; or
1 cement, 4 sand, 6 broken stone (3 inch to 2 inch mesh).
The gap space with gravel (pebbles all sizes from 2 inches to s
^ inch) is about 35 % and with broken stone if graduated to include
the same sizes it is about the same. If the broken stone is larger
(3 inch to 2 inch mesh) the gap space will be larger and it is
therefore not so economical in cement.
The weight of materials in Ib. per cubio foot may be taken as
follows :
Cement 90, sand 90, gravel 110, concrete 135. It must not be
overlooked when estimating that the sum of the volumes of the
constituents before mixing is some 40 to 50 % greater than the
volume of the resulting concrete.
A suitable natural mixture of gravel and sand can often be ob
tained on site.
To make the concrete, the cement is first well mixed with the
sand, dry, then water is added, mixing all the while until the con
sistency of moist earth is reached. The aggregate of broken stone
or gravel is then added and the whole well mixed.
The filling should be done in 6inch layers which must be well
rammed until a layer of water appears on the surface.
If the work is well done a compressive stress of about 1 700 Ib. /
sq. in. can be counted upon in 30 days after setting, and it is there
fore quite unnecessary to use richer mixtures.
It is most important to see that no earth gets mixed with the
concrete, as its strength may thereby be seriously weakened.
The concrete should extend six inches or so above the ground
and the top should be sloped ofr a little to prevent rain from settling.
The top surface should be faced with a 1 : 2 cement mortar.
Compound Channel Poles. For larger loads, compound
channel poles may sometimes be found useful. A simple example
will be considered, consisting of two of the channel iron poles de
scribed above arranged as in Figs. 72 and 73 (a) with 12 inches
between backs of 6 inch by 3 inch channels.
The information in the following Table XV. will be required in,
addition to that given in Table VIII., page 71.
IRON AND FERROCONCRETE POLES
Y <='
1 4,5
POUND INCHES.
FIG. 73. Compound channel iron pole.
10
146
OVERHEAD POWER LINES
TABLE XV.[B.S. Spec. No. 61924.]
Channel
Section.
Distance of C.G.
from Back of
Channel,
ins.
h,
ins.
,
ins.
4X2
599
24
31
5 X 2
773
25
38
0x3
890
25
38
Strength of 6 inch x 3 inch Twin Channel.
J Ml = J VVl + A.S. 2 (Fig. 72).
Loss of area due to two ~inch brace bolts (or rivets)
== 5 X '38 X 2 = 38 sq. in.
.. Reduction in J^ = 38 X 66 2 = 165
and Reduction in A.S.* = 38 X 45 2 = 769,
both values being approximate only.
.. Nett J COi = (2825 165) + (365 X 511 2  769)
= 9022 incli units for each channel
.. Total J cc for compound section
Allowing
9022 x 2 = 18044
18044
~~ 6
/= 60 000 Ib. / sq. in. and F. of S.
3007 inch units.
25.
a .' a/r frr 60000x3007 _. AAr . .
Safe M = /Z CCi =  ^=  = 722 000 Ib.ms.
Working similarly it will be found that M = 576 000 and 388 000
Ib.ins. for the 5 inch by 2 inch and the 4 inch by 2 inch twin
channels respectively.
The Moment of Resistance diagram is given in Kg. 73 (b), from
which it will be seen that if the pole is set 5 feet 6 inches in the ground
it will be suitable for a lateral load applied 2 feet from the top of
722 000
245 x 12
2 460 Ib.
It is therefore approximately equivalent in strength to a 32 feet
by 9 inch wood " A " pole.
IRON AND FERROCONCRETE POLES 14/
Ratio of Lateral to Longitudinal Strength.
r . . . . 7 , 7 722000 010
. . (a) Available =,. _ _ ^ = 212
6 ins. X 3 ins. j v ' 170 160 X 2
[ (6) Utilised 212
576 000
5 ins. X 24 ins. ^ ^OOO^
11
(a)
114000X2
388 000
4 ins. X 2 ms. , 260000
60 770 X 2
The design does not therefore make the most economical use of
the sections. The distance between backs of channels may be in
creased to 20 inches nearly without exceeding the ratio of lateral
to longitudinal strength of 4 to 1 allowed by the regulations, and the
working load may be increased accordingly.
The above calculations assume the two members to be parallel
to one another, but in practice it is usual to incline them towards
each other, mainly for aesthetic reasons. The moment of resistance
diagram for the case in which the members are inclined, with .12
inches between backs of channels at the ground line and 8 inches at
the 'top is shown dotted in Fig. 73 (6). With this arrangement it
will be seen that the working load must be reduced to 
294
= 2 170 Ib. We will, however, continue our consideration of the
parallel arrangement.
* Bracing Required Above Ground. A suggested arrange
ment of the bracing is as follows : on each side of pole five diagonals
in the 4 inch X, 2 inch bay, four in the 5 inch X 2 inch and two in
the 6 inch X 3 inch. The free lengths of the diagonals will then be
about 255 inches in the 4 inch X 2 inch bay, 295 inches in the 5 inch
X 2 inch and 295 inches in the 6 inch X 3 inch. Pig. 73 (a) shows
the diagonals on one side only. The diagonals must be designed
to take the shearing load, viz. 2 460 Ib.
Distance between centres of rivets is 9 inches approximately.
Therefore the LOAD ON DIAGONALS
2460
2 cos oc 2x9
148 OVERHEAD POWER LINES
We will use 1 inch X 1 inch X  inch equal angle, for which the
least radius of gyration k = 29, .
. 2=29*
" k 29 iU1 '*
Using Eider's formula (see p. 74) and a E. of S. of 25, the SAFE
COMPEESSIVE LOAD
12 . 10 7 . A 12 X 10 7 X 687
(1017)
 800
Size of Rivet. In addition to shear stress there will be a
tensile stress in the rivet due to pulling up on contraction, and also
a certain amount of stress due to bending. The F. of S. should there
fore be based upon the elastic limit.
If d = dia. of rivet required, we have
" X 27 000 = 4 030 X 25,
4
and d = 69 in.
A i^.inch rivet will therefore be necessary, and consequently
the calculations given above for strength of channels should be
repeated, as they were based on inch rivets.
The reduction, in strength however, will only be about 4 % and
it is proposed to neglect it here.
Tensile Stress in Diagonals. The alternate diagonals
will be in tension. Allowing for loss of area due to inch rivet,
the nett sectional area = 526 (688 X 1875)
= 526 129 = 397 sq. in.
.. TENSILE STRESS = ~^r= 10 150 Ib. / sq. in.
097
The B.C. require a P. of S. of 25 on the Elastic limit i.e. a limiting
stress of
36000 _ . ....... .
 = 14 400 Ib. / sq. in.
a'O
The 1^ inch X 1 inch X fVinch diagonal is therefore amply
strong enough both in tension and compression.
Buckling Strength of Channels. It is advisable to arrange
IRON AND FERROCONCKETE POLES 149
tlie diagonals alternately on the two sides of pole to reduce the free
length of the channel considered as a strut. 4
Consider, for example, the lower end of the 4 inch X 2 inch
channel.
The free length = 24 inches approximately and the distance
between the centres of gravity of the two channel sections = 98 in.
The B.M. at the working load = 260 000 Ib.ins. (see Fig. 73 (6)),
therefore the vertical compressive loading
, = 260000 = MB001bi
y*o
To this should be added one half the dead weight load of the
pole, pole fittings and conductors, which would be about 2 000 Ib.
in all, therefore the total compressive loading = 26 500 + 1 000 =
27 500 Ib. For 4 inch X 2 inch channel, A = 2085 sq. ins. and k
(lesser value) = 703. !
.. BUCKLING LOAD, P = (46 000 166,) M fc085 1
\ * '
= 840001b.
and the JF. of S. = = 306.
Forces Below Ground. It is the usual practice to set poles
of this type in concrete. For calculation purposes the concrete is
supposed to be a homogeneous body with a definite elastic modulus,
in which case the stress distribution will be as represented by the
area OABOD in Fig. 74 (b], page 150, the pole being supposed to
pivot about the point 0, its centre point below ground.
If / is the maximum compressive stress in the concrete, then
the areas of the triangles OAB and OOD will be each equal to
/ X 6.x 33 lb T]ie Q^ of tliege triang i es w iu be f X 33 inches
Zt
from 0, therefore the MOMENT OF RESISTANCE OF CONCRETE
= /.X 6X33X44^
2
This must be equal to the BENDING MOMENT DUB TO THE LOAD
which
= 2 460 X (294 + 33),
. 2 460 X 327 X 2 . OK ,, .
*= 6X33X44 =18B)b./^.m.
150
OVERHEAD POWER LINES
The concrete will stand 1 700 Ib. / sq. in. and there is, therefore,
a high factor of safety in this respect.
Bracing Required Below Ground. The direct horizontal
loading at the extreme points A and C (Fig. 74 (&)) will be 185 X 6 =
1 110 Ib. per inch run due to the reaction of the concrete. Since the
horizontal forces must themselves balance as well as their moments
1110
LBS
FIG. 74. Foundation portion of compound channel iron pole. (See Pig. 73.)
we must also consider the pole top loading which may be taken as
., , , , 2 460
nform load of = 37 Ib. per inch run. Therefore the total
t 4=^1110+37=] 147 Ib. and at 0=111037 =
per inch run. The loading decreases uniformly down to
IRON AND FERROCONCRETE POLKS
If the total load on O.A. = W + ^
x z P .x
then the load on. a length O.E. = Wj$ +
.. SHEARING FOECB AT E (Fig. 74 (b)).
= W
i i in v OQ
Now W = 1U 2 X 66 = 18 300 Ib.,
.. Substituting known values in above equation the shearing force
at any point x inches above O
 is 300 (i 
~ V 33V "2X S3
At the point 0, the maximum values occur, viz. 18 300  .1. 230
= 19 530 Ib. from right to left and 18 300 1 230 =: 17 070 1I>.
from left to right below 0. TIi.e shearing force diagram is shown
shaded in Pig. 74 (6).
It will be obvious that we need only to consider the lower value,
Now if the pole is set in concrete and the interior Hj>aoo are well
filled and rammed, the braces will be relieved of thcso very largo
shearing forces, but we will assume that the interior is empty. A
possible method of bracing to meet this latter condition in shown in
Fig. 74 and consists of two 6 inch X J inch plates at ground Him,
two 6 inch X ;} inch plates at butt, two 12 inch X Jjr inch plates at
centre ; and four 3 inch X 3 inch X f inch diagonals (two only
shown in Fig. 74).
The free length of diagonals = *J&~~\ 22 2 = 24 inches approx.
For 3 X 3 X  inch equal angle, A = 211 and k ~ 58
! = ?* 413
k 58 '
.. BUCKLING LOAD = (46 000 ICO X 413)2ll
= 82 600 Ib.
The working load on each diagonal
e Q
01 &<
y
826QQ
OVERHEAD POWER LINES
Rivets. Assume froch rivets are used, and let n be the numbi
j%
required, then  X n X 27 000 = 22 800 X 25, and n = 48.
Therefore five finch rivets would be required. Ib is, howeve
more convenient to use four rivets, and these must be *f inc
in diameter.
Stress in Diagonal in Tension. Allowing for loss of are
due to one ifinch rivet, the effective crosssectional area (
diagonal
= 2 11  (375 X 8125) = 1 806 sq. ins.
99 00
.. TENSILE STRESS = 12 600 Ib. / sq. in.
1* oOo
This is well "below 14 400 Ib. / sq. in., the value permitted by th
E.G. Regulations.
The Factors of Safety so determined are on the pessimistic side
since, although we have neglected the somewhat eccentric loading c
the diagonals, we have also neglected the help given by the incl
plates.
Many other arrangements of the bracing will suggest themselves
but it will be clear that if the pole is set in the ground in such i
way that stability is to be ensured by horizontal reactions, then th
shearing forces in the foundations require special consideration.
The plates at the ground hne will not help much except to stiffei
up the structure, especially during handling and erection, but th<
two 12inch plates at the centre will take an appreciable share of th<
shearing load.
Ground Reactions. Assume a concrete block 24 inches X 45
inches X 72 inches, as shown in Fig. 75.
First consider the ground reaction under the block. The maxi
mum rupture intensity at a depth of 72 inches = =
146 Ib. / sq. in. (Fig. 91, p. 181). The approximate weight of pole
and conductors will be 2 000 Ib. and of the concrete block 6 000 Ib.
therefore the direct compressive stress
jj The load diagram is then as shown in Fig. 75 and the MOMENT OB
]! RESISTANCE due to this load
I 146 793
!: = o X 24 x 42 x 4 = 278 000 Ib.ins.
= nW Ck
I8150 o
793lbi
against overturning
&
2 093 000 ^ 43
"
^jtr
5,5Ss^
154 OVERHEAD POWER LINES
the block (shown dotted in Fig. 75) to take advantage of the superin
cumbent earth pressure thereon.
FerroConcrete Poles. Ferroconcrete poles have not beei
used to any extent in this country, but they have been used a goo<
deal on the Continent and in America. Their great weight is i
disadvantage, some designs being 3 to 4 times as heavy as equiva
lent wood or iron poles, although the ratio has been brought down t<
2 in some cases, notably in the " Harriot " design of pole, which i
manufactured in this country. They require more care in handling
than wood or steel poles and are frequently stressed more during
transport and erection than they are likely to be subsequently ii
service. They have to be constructed on or near the site, and this
combined with their great weight, makes their use impracticabl<
in difficult country.
At the present time they cannot compete with wood poles iron
a first cost point of view, but since they are practically everlasting
and their maintenance charges negligible, and as the supply o:
timber is unlikely to keep pace with the demand in the near future
they may have to be seriously considered for distribution purposes
in competition with iron.
The design and construction of ferroconcrete poles should nol
be lightly undertaken, unless the engineer has had experience oJ
ferroconcrete work and has time to consider the matter carefully,
The concrete mixture should consist of 1 Portland cement, 2
sand, 4 broken stones or gravel (pass finch mesh but retained by
Y\inch mesh). When well made, the crushing strength of such a
mixture is about 1 000 Ib. / sq. in. in 7 days, 2 500 Ib. in one month;
3 000 Ib. in three months and 3 500 Ib. in six months. The weigh!
is about 140 Ib. per cubic foot.
Round reinforcing bars are invariably used, and, within limits,
a large number of small bars are better than a few larger ones owing
to the larger surface they provide for adhesion between steel and
concrete. Angle and tee sections must not be used, as although they
provide a large surface area compared with their crosssection it is
practically impossible to ensure that the concrete is packed securely
in the corners. The reinforcement should be covered with concrete
to a depth which need not exceed 1 inch, but should be at least
equal to the diameter of the bar. The bars should be free from
loose rust.
To illustrate the principles involved, a simple example of a
IRON AND FERROCONCRETE POLES 155
3?:o. 70. Ferroconcrete pole.
156 OVERHEAD POWER LINES
successful design, shown in Fig. 76, will now be briefly considered.
The following assumptions are made :
(1) The pole is subjected to pure bending and the total compres
sive load on one side of the neutral axis is equal to the total tensile
load on the other.
(2) Plane sections remain plane after bending.
(3) For the concrete, as well as for the steel, the stress is pro
portional to the strain.
(4) There is no slip between the concrete and the steel. This
is true except in the case of very short columns ; also the coefficients
of expansion of steel and concrete are about the same, and therefore
no appreciable stresses are set up by ordinary changes in tempera
ture.
(5) The whole tensile load is taken by the steel reinforcement.
(6) The area of the reinforcement is so small that we may assume
the stress constant over it.
The following constants will be taken :
Ultimate tensile stress Steel 65 000 Ib. / sq. in.
,, Concrete 250 ,,
Ultimate compressive stress Steel 55 000 ,,
Concrete 2 500
Modulus of elasticity Steel 30 . 10 6
Concrete 2 . 10 G
Safe maximum adhesion between concrete and steel 100
In structural work it is usual to design for a Factor of Safety of
2 for steel based on the elastic limit, and a F. of S. of 4 for concrete
based on the ultimate compressive stress. We will assume, however,
that the bending theory holds up to the ultimate stress, and allow a
F. of S. of 35 on the structure as a whole, as required by the E.C.
Regulations.
The following notation will be used :
Tensile stress Steel f st Concrete f ct
Tensile load ' T s
Compressive stress / Concrete f cc
Compressive load C s C c
Modulus of elasticity E s E c
Area of cross section A* ,, A f
IRON AND FERROCONCRETE FOLES
157
Fig. 77 (a) shows a crosssection at the ground line of the pole
under consideration. For simplicity we will neglect the web portion
and the eight ^inch rods.
To calculate the strength of the pole we must first find the neutral
axis, NN'. Let this be x inches from the edge of section on com
pressive side.
Then, as there is to be no slip, we have (Fig. 77 (&))
Maximum compressive strain in concrete A A 1 AS x
Compressive strain in steel CC 1 CS x 13'
i.e.
Now
E c
Isc
E, _ 30 . 10"
E ~ 2 . 10 e
15,
13
x
/fi/i>/sjr,
(a.) Section.
(c) Stress
and limiting / to 2 500 Ib. / sq. in.,
/ = 15 X 2 500 (l  ) = 37 500 
Ib. / sq. in.
\ * f UU
Area of Steel = 2 X ~[Y) + (^} j = 16 sq. ins.
.. TOTAL COMPRESSIVE LOAD ON REINFORCEMENT
= (L = 16 ^37 500  4
158 OVERHEAD POWER LINES
Similarly tensile stress in steel on tension side
=37 600=37 50oi 7 _ l
X
,. TOTAL TENSILE LOAD ON REINFORCEMENT
_ 37 500 = IMo _ 60
We must now find the compressive load on the concrete. The
stress at the outer edge of iange = 2 500 Ib. / sq. in. and at the
, % _ g.gs
inner edge = (2 500 X  ) Ib. / sq. in.
& \ x J ' i
The average stress is the mean of these two values, viz.
2500 I
 _  = 500  Ib. s . in.
Area of concrete flange = 35 X 12 = 42 sq. ins. (neglecting small
loss of area due to reinforcement).
.. TOTAL COMPRESSIVE LOAD ON CONCRETE
Ib.
422 500  * = 105 000
X J \ X J
Now by assumption C s + G = T s , i.e.
60 000  _ + 105 000 _ = _ 6 o 000.
a; xx
Whence x = 615 inches.
4.0 7KA
/ = 37 500  =^p = 29 570 Ib. / sq. in.
O, = 29 570 x 16 = 47 400 Ib.
f at = !^? _ 37 500 = 76 500 Ib. / sq. in.
CThis stress is too great, but we will continue the calculations and
^w for this later.)
IRON AND FERR.OCONCRETE POLES 159
TI = 76 500 X 16 = 122 500 Ib.
f e at outer edge of concrete flange = 2 500 Ib. / sq. in.
$ _ 3.5
/ c at inner edge of concrete flange = 2 500 X 
Q5
K.TK _ 9.K
= 2 500 X * = 1 080 Ib. / sq. in.
615 ' A
O c = 42 x 1 080 + 42 x 2 5 7 l 8 = 75 100 Ib.
2i
(See Fig. 77 (c).)
Total compressive load = C a + G G = 47 400  75 100 = 1.22 500 Ib.
Moment of Resistance. Let the centre of pressure of the
compressive loading be y inches from the outer edge of concrete
flange, then taking moments about this edge, we have, dividing the
stress diagram for the concrete into i octangular and triangular
portions (Fig. 77 (c)),
Q.K 1 A r>(\ <>.
122 500y = 47 400 X 13 ~ 1 080 X 42 X . + ~  X ^ X ' 
22 o
= 61 600 + 79 400 j 34 800.
W1 175800 , , . ,
Whence y = j^^ = 143 inches.
.. Centre of compressive loading = 015 ~ 143 = 472 inohoa from
neutral axis.
Now centre of tensile loading = 20 13 615 1255 inches.
from neutral axis.
.. Arm of resisting couple = 472 f 1255 = 1727 inch OR.
and MOMENT OF RESISTANCE = 322500 x 1727
= 2 120 000 Ib.ijw.
But this based on f at = 76 500 Ib. / sq. in., whereas the ultimate
strength is assumed to be 65 000 Ib. / sq. in. only. The moment of
resistance, therefore, must not exceed
2 120 000 x  1 800 000 Ib.ins.,
and allowing a F. of S. of 35 the BENDING MOMENT must not exceed
1 J
160 OVERHEAD POWER LINES
The gross maximum working load at a point 2 feet from top of
pole (i.e. 305 feet from ground level)
305 X 12
Alloy/ing for wind pressure on pole, the nett safe working load due
to wind on wires = 1 410 140 = 1 270 Ib.
The pole is approximately equivalent to a 40 foot/ 8 inch " A "
pole. In the above treatment we have neglected the direct com
pressive loading due to the dead weight of the pole and conductors,
which would together be about 5 000 Ib. The crosssectional area
of the concrete = 1165 sq. ins. and the compressive stress, if as
sumed to be taken wholly by the concrete, would be increased by
5000 , K11 . .
=:451b./sq.m.
On the other hand, owing to the limit imposed by the strength
of the steel on the tension side, the maximum working compressive
stress in the concrete due to bending is  X =^~K = 607 Ib. / sq.
O"0 (O'O
in. only, instead of  715 Ib. / sq. in., which is the safe maximum ;
O'O
moreover, the eight Jinch rods have been neglected, and at the end
of six months the strength of the concrete will have increased by
40 %.
Stress in Concrete on Tension Side. We have neglected
the tensile strength in the concrete in our calculations, but if, as we
have assumed, there is no slip between the steel and the concrete,
there must be a tensile strain in the concrete equal to the tensile
strain in the steel.
,, , 65 000 . .
If f,t = Q ft / sq. in.
4.1. x 65000 2.10 6  , nl , .
then / ot = gg x ^Y QQ = 1 240 Ib. / sq. in.
This is an extreme figure, since in the initial stages the concrete
takes a part of the tensile load.
Now the ultimate tensile stress of concrete is only about 250
Ibs. / sq. in. Hence it is evident that in service the concrete
cracks on the tension side, but owing to the adhesion between the
"Fia. 78. Eerroconcrete pole.
Overlapping joints in the reinforcement are shown to introduce the calculations
involved, but welded joints would invariably be used in modern practice.
11
162 OVERHEAD POWER LINES
steel and the concrete, the failure consists of a very large number of
small hair cracks extending along the whole length of the pole.
Moisture from the air enters these cracks and automatically seals
them by acting on minute unhydrated particles of cement. Thus
the steel rods are protected from corrosion.
STRENGTH OF JOINT (Fig. 78).
Length of Joint = 39 ins.
Centre of Joint = (39 195 2)12 = 210 inches from point
of loading
MAXIMUM BENDING MOMENT AT CENTRE OF JOINT
210
= 515 000 X STT = 296 000 Ib.ins.
obu
Arm of couple =11 inches approx.
.. Tensile load on joint = = 26 900 Ib.
Surface area of reinforcing' rods = 39 X TT (f f f ( / F ( T V)
= 292 sq. ins.
.. Max. longitudinal stress between steel and concrete
26900 .
= "292" = ^ Sq ~' m *
The safe maximum = 100 Ib. / sq. in.
163
CHAPTER IX.
ANGLES AND TEEMINALS.
Angles. We will calculate the stress in the stay wire, etc.,
making the following assumptions :
(!) Three 05 sq. in. copper conductors and an earth wire ar
ranged as in Eig. 26, p. 45.
(2) 34: foot / ll inch pole buried 6 feet.
(3) Span length, 250 feet.
(4) Angular deviation, 10 degrees.
Under these conditions the lateral load on each insulator
= P f 2 . T sin  (p. 60)
A
= 1775 + 1 457 X 174 = 431 Ib.
The total load due to four wires = 431 X 4 = 1 724 Ib.
The centre of pressure is approximately 336 2875 = 30725
inches from ground level.
The bending moment on the pole at the ground level due to wind
pressure on the pole itself = 31 600 Ikins., which is equivalent to
e\ t C\f\r\
a load of ~~j~ = 103 Ib. at a height of 30725 inches.
{The wind load on the pole fittings is small and may be neglected.)
The total LATERAL LOAD is therefore
P=1724 103= 1827 Ib.
Stay (Fig. 79, p. 164). In order to keep the stay wire well clear
from the line conductors it will be advisable to fix it a little below the
centre of pressure, say 36 inches from top of pole.
It will not usually be necessary to bother about any small dif
ference between the centre of pressure and the point of attachment
of the stay, but we will allow for it in this case to illustrate the prin
ciples involved.
164
OVERHEAD POWER LINES
ANGLES AND TERMINALS 165
If PJ is the horizontal component of the reaction clue to the stay
wire it must have such a value as to prevent any deflection of the
pole at the point of attachment.
It must be emphasised here that as far as the load on the stay
is concerned no allowance can be made for the strength of the pole
itself, which acts simply as a strut. The pole deflects when stressed
due to lateral loading and if this occurs it means that the stay wire
or its anchorage has failed.
It can be shown that to prevent any deflection of the pole, P x
.0 rr .
must be equal to (  }P, in which H is the height of pole to
centre of pressure and S the height of pole to point of attachment of
stay. In our example
3X 30725
2X300
= 104 X 1 827 = 1 900 Ib.
Now whether we use a stay or a strut, the further from the pole
we are able to fix it the less the loads to be dealt with.
We will assume for purposes of calculation that 15 feet spacing
is available. Then the
A/25 2 4 15 2
TENSION IN STAY = v ,~T  x 1 900
15
9Q9
= Lf x 1 900 = 3 700 Ib.
15
Referring to Table XVI., p. 170, we find that a 7/16 stay wire
will meet the requirements.
25
COMPRESSIVE LOAD ON POLE = x 1 900 = 3 170 Ib.,
15
to which must be added the dead weight of the conductors, pole
fittings and of the pole itself, which would be about 1 850 Ib., making
a total of 3 170 + 1 850 = 5 020 Ib.
The buckling strength can be checked as explained later for
terminal poles (see p. 172).
Intensity of Ground Pressure under Butt. Taking the
butt diameter as 12 inches this
5 020 X 4 , , K ,
= = 445 n>. / si m .,
166
OVERHEAD POWER LINES
431 LB3
CENTRE OF PRESSURE
P
which is just below the safe maximum (see Fig. 91, p. 181) at a depth
of 6 feet. But as it is most important to avoid settlement it will
be advisable to distribute this load over an area of at least 2 sq. ft.
bv means of a block of concrete or creosoted timber.
25
The stay wire makes an angle of tan" 1 = 59 with ground level,
3 700
so if the stay block is buried 4 feet, an area of ^^ = 231 sq. ft.
is nec essary (see Kg. 91, p. 181).
A stay block 9 inches X 4 inches X 3 feet 6 inches long is sug
gested.
Strut (Fig. 80, p. 164) shows the usual form of construction.
Struts are seldom used. They are rather unsightly and more
expensive than stays and
in the survey of the route
every endeavour should
be made to render struts
unnecessary. They may
sometimes be advisable,
however, if stays are likely
to be attacked by im
purities in the atmosphere,
especially near chemical
works.
In this connection it
may be noted that a
galvanised high purity
iron stay wire has recently
been standardised which
_. . is more durable than sal
lie. 81. . . 5
vanised steel wire, (bee
B.S. Specification 1831927.)
Taking the same conditions as assumed above for the stay, we
will suppose the strut to be fixed 12 inches below arm bolt.
Bending Moment on Pole at Weakest Section. This
will be where the strut is attached (Fig. 81)
= 862 x 20 + 431 X 62 = 44 000 Ib.ins.
Diameter of pole at this point would be about 85 inches.
JujL
j&.$/$M*>'> f * v >''ffr/i.w'<y.WJk
ANGLES AND TERMINALS
.. Moment of resistance
800 X rr X 86* =
_ 
32 ~ 32
There is therefore ample strength here. The horizontal reactioi
due to strut must
 41827
2X 283
= 113 X 1 827 = 2 060 lb.,
neglecting wind pressure on strut itself.
Assuming strut to be fixed in ground 8 feet from pole, tho upwaix
pull on pole
= 2 060 X H = 6 070 lb.
96
Allowing for dead weight of pole, etc., the NETT UPWARD PIILI
ON POLE FOUNDATION
== 6 070 1 850 = 4 220 lb.
Cross blocks will, therefore, be necessary to HOLD Tim POL
DOWN. If they are fixed 4 feet below ground level their arc
A OOQ
must not be less than f = 338 sq. ft; (see Fig. 91, p. 181).
A suggested arrangement is shown in Fig. 80 employing tw
8 inch X 4 inch X 3 feet cross blocks and two 2 inch X 12 inch >
2 feet kicking blocks.
The upward pull on strutted poles is a good deal greater than i
usually supposed.
Compressive Load on Strut.
TH3 = 2060x^3=2060x1? =0420 11,
Assuming the strut to have one end fixed and tho other pivote
and neglecting the restraint due to the buried portion and to tl
tie, bolt, the mean diameter required will be given by tho equation
r> 2257r^/ _ 225 X 7r z X 12 X 10 X TT X D* _ r , 9ft
B==  ____ ^ 64 X 295* X 12 2 ' ~" '"" X '
__ 6 420 X 35 X 64 X 295 2 X 12"
Wlience ~
168 OVERHEAD POWER LINES
Length, of strut required is about 30 feet, allowing a buried depth
of 4 feet 6 inches. A 30 foot / 8 inch pole is suitable.
Ground Pressure under Butt of Strut. The strut itself
weighs about 550 lb., and assuming for simplicity that the whole of
this acts in the direction of the strut the total load on the ground
tinder the strut = 6 420 + 550 = 6 970 lb.
The symmetrical ground pressure at a depth of 4 feet must not
exceed
4~ = 329 lb. / sq. in. (Fig. 91 p. 181).
TT X 9  35 2
Area of pole butt =    = 69 sq. ins. approx., therefore
the ground under pole will only support 329 X 69 = 2 260 lb.
Consequently cross blocks must be provided to take a load of 6 970
2 260 = 4 710 lb. A creosoted wood or concrete block about
18 inches square, under the butt, would meet the case, but it is pre
ferable to fix a cross block, as the strut is then well anchored and
will act as a stay as well, should circumstances ever require it to
do so.
If the cross blocks are fixed with their under surface an average
4710
of 3 feet 6 inches below ground level, an area of _  = 165 sq. ft.
2 860
will be necessary.
Two 8 inch X 4 inch X 3 foot blocks are suggested.
Strength of Scarf Joint. The length of scarf on the strut
will be about 2 feet and it is therefore advisable to use two bolts,
one at bottom end of scarf and the other about 12 inches above.
The vertical load taken by the scarf joint = 6 070 lb.
If the strength of the joint so constructed is checked as shown on
page 122 it will be found to be distinctly weak, depending as it does
simply on two bolts, and the help given by friction.
The moment of resistance of the pole itself can only come into
play if the strut foundations give, and this is forbidden by the
regulations.
The fact that this type of joint seldom fails can only b ascribed
to the rare occurrence of the hypothetical loading conditions.
To make a really satisfactory job and provide a factor of safety
of 35 an oak block should be fitted as in the case of an " A " pole,
or the strut should be let into the pole for an inch or so at the top.
ANGLES AND TERMINALS
16!
The pole invariably has sufficient margin of strength, to permit o
this.
Use of Rutter Poles at Angles. Kef. to Fig. 65, page 132.
shows than an 8inch Rutter pole has a safe working load of about
2 400 Ib. at a point 26 feet from ground level and would therefore
be quite suitable for the conditions considered above, thus obviating
the use of stay or strut.
Rutter poles are now being largely employed for angles, and they
are particularly useful in cases where space is limited.
In all cases angle (and terminal) poles should be given a slight
" rake," i.e. an inclination away from the pull to allow for the small
" give " in the foundations which occurs in the initial stages when
the load is applied.
Terminals. We can seldom use cap fittings on terminal poles,
and it is advisable to allow greater clearances between conductors,
therefore somewhat higher poles will be
required if the triangular arrangement
is maintained. To obviate the use of
longer poles the three conductors can be
placed in the same horizontal plane, but
this does not make such a neat job. We
will base our calculations on a 36
foot / ll'l inch pole buried 6 feet, the
stay wire being fixed 26 feet from ground
level and the conductors arranged as
shown in Fig. 26, page 45.
The maximum longitudinal pull on
the pole under basic loading conditions = 1 457 X 4 = 5 828 Ib.
If the stay is fixed in the ground 15 feet from foot of pole, the
TENSION IN STAY WIRES (Fig. 82)
T
5 828 A/26 2 + 15 2
15
= 11 660 Ib.,
and the COMPRESSIVE LOAD ON POLE
26
5 828 X
15
lOHOlb.,
to which must be added the weight of pole, pole ironwork and of
half a span of iceloaded conductors and earth wire. This 'will
approach 1 500 Ib.
170 OVERHEAD POWER LINES
Therefore TOTAL COMPEESSIVE LOAD ON POLE
= 10 110 H 1 500 = 11 610 Ib. approx.
To provide the required factor of safety of 25, the breaking load
of stay wires must not be less than 11 660 X 25 = 29 150 Ib.
Table XVI. gives particulars of the most common sizes of stay wire.
TABLE XVI. Particulars of Stay Wires and Stay Rods.
Breaking
Safe Work
Weight
Load,
ing Load,
per Foot,
Ib.
Ib.
Ib.
Galvanised Steel Stay Wire 7 / 08
2450
980
120
^
4/16
5 COO
2240
274
1 B.S. Spec.
7/1C
9800
3920
479
f 183, 1927.
19/16
26000
10 640
1310
J
StayR
ods wi
liTigl
tenets in.
H
10G50
15 900
4260
6 360
] British
8
21 000
29100
8 400
11640
> P.O.
J Standard.
The figures in the table assume a breaking stress of 70 000 Ib. /
sq. in. for the galvanised steel stay wire and about 52 000 Ib. / sq. in.
for the galvanised wrought iron stay rods (at bottom of threads).
It is not advisable to use wire of greater tensile strength, as it
deteriorates more rapidly.
Instructions for Attaching yStrand 016 Dia. Stay
Wire to Thimble. Bend the stay wire to form, two knees
7 inches apart, the first of these knees being 23 inches from the
end of the wire.
Bend the wire between the two knees round the thimble, using
' ' O
the stay tool to draw it close into the groove. Unstrand the free
end, straighten out the wires, pick out one end for the first lap, and
loosen the tool whilst the wire is passed underneath it, again grasp the
remaining wires with the tool and place them symmetrically parallel
with and around the main strand, so that they will bind into it
without spoiling its circular shape. Grip with the tool and revolve
the latter with the free wire under the hook on the thimble side of'
the tool. This wire should make eight laps.
Treat the other wires the same way, as shown in Fig. 83.
The projecting short ends of each wire (which should not be more
than \ inch long) must be worked in by grasping the splice with the
ANGLES AND TERMINALS
17
tool (the ends being within the hollow of the tool) and turning th
tool over each end until it is worked in.
Method of Attaching 7Strand 016 Stay Wire to Pole.
Lap twice round the pole, secure by half a dozen No. 4 S.W.G
staples and finish off the loose end on to the standing part in th
Stavj Tool.
Qir  8 La,o3 around main., binding in 6 loos*
 7
 7
Total 49 Lajos
.. 3
ITiG. 83. Making off 7 strand '16 inch dia. stay wire on thimble.
manner described above. Alternative methods are shown in Fi
79, p. 164.
For a breaking load of 29 150 Ib. it will be seen that three 7/v
stay wires are required, or their equivalent.
(For symmetry, using an H. Pole, four 7/16 stay wires won
probably be used in this case.)
172 OVERHEAD POWER LINES
If, however,, a distance of 26 feet from the foot of the pole is
available, the load on the stay will be reduced to
.
26
In this case three 7/16 stay wires will give a F. of S. of
. .  .. 356, and two such wires a F. of S. of 237 only, which
is hardly sufficient, but in practice they might possibly be made to
satisfy the requirements by fixing them a little further from the pole.
The direct compressive loading on the pole will in this case be
reduced to
5 828 + 1 500 = 7 328 Ib.
The effects of this compressive loading will now be considered.
Resistance of Earth under Pole to Direct Compres
sion. The intensity of pressure on the earth under the pole when
the stay is 15 feet from pole
11610 .
103 Ib. / sq. in. approx.
From Fig. 91, page 181, it will be seen that the maximum safe
intensity of pressure at a depth of 6 feet is about 58 Ib. / sq. in.
It will therefore be necessary to distribute the pressure by means
of a block of creosoted wood or concrete.
Strength of Pole to Buckling. It was pointed out on page
119, when considering the buckling strength of the compression
member in. " A " poles, that no great precision could be claimed for
the calculations. The same remark applies here.
It is difficult to procure poles perfectly straight and to erect them.
exactly vertical. The loading is not concentric, and the conductors,
sarth wire and stays are not all attached to the pole at the same
^oint. Moreover, the " end conditions " and the " effective "
aigth of the pole considered as a strut can only be guessed at.
Lateral deflection of the pole in strong winds further complicates
lie problem, but this can be obviated by splaying out two of the
erminal stays as indicated in Fig. 84.
It will be realised, therefore, that exact calculations are impos
ible, taking all the relevant factors into consideration, but Euler's
ANGLES AND TERMINALS 173
formula for a strut hinged at both ends, taking values for E, J and L
as defined below will be found to give results agreeing closely with,
experiment.
If E = Modulus of elasticity = 12 X 10 6 Ib. / sq. in.,
J = Moment of inertia (in inch units) of the crosssection of
the pole about half way up,
L Overall length of pole up to point of loading (in inches)
(the reaction of the ground to the buried portion being
neglected),
B BUCKLING LOAD IN POUNDS,
7T 2 . E . J
then D = ^5 .
FIG. 84.
In our example
Mean diameter of pole = ^ = 10 inches approx.
77 . 10 4
= 491 inch units.
__
64 64
L = (36  4)12 = 384 inches.

L * 3342
(a) With stays fixed 15 feet from pole
39400
(b) With stays fixed 26 feet from pole
The pole selected would, therefore, be quite suitable for case (b),
but is not quite strong enough for case (a}.
It is to be remarked, however, that the minimum specified dia
meter of the standard pole has been assumed. A consignment will
174
OVERHEAD POWER LINES
POLES
Trussing TacMe Stng/e, comp/e/&.
consfsfo of i TrtrSS fl//j$ * /
B/ocfa for " " = 8
Truss ffgi/s A //TC/I .
Tte Boffe 
G. /. Tub/ng /' ieng/hs 
Wasfie
D/a/?>efer of Po/es <* c/s's/Jn
oehveen cen/fes fnuaf he
iSpeetfject tvJien ofe/er/ng
'ftng <?/ ' flo/e
Pio. 85. Typical terminal "H" pole. (Por details of pole fittings, see Fig. 8G.)
ANGLES AND TERMINALS
il
176
OVERHEAD POWER LINES
invariably include some poles with, diameters appreciably above the
minimum and these would naturally be earmarked for terminals and
angles.
In practice, wayleave considerations tend to shorten up the
stays and it may be found necessary to use nonstandard single
poles of large diameter or, preferably, " H " poles.
The latter are more desirable from electrical considerations as
the greater space available makes it more convenient for dealing
with conductors and stay wires and
for fixing cable terminating boxes,
switchgear, etc.
Figs. 85 and 86, pages 174 and
175, show a* suitable design for the
line under consideration.
Stay Anchorages. It is
often assumed that the resistance
offered to a stay block is equal to
the weight of earth contained in
the frustrum of a pyramid of
which the smaller base is the stay
block itself (assumed horizontal) the side faces make an angle <f>
with the vertical and the larger base is the ground surface.
. The weight of such a frustrum of earth (Fig. 87) is given by the
following expression :
W =
j 2d tan 0) + Ib
2d tan
2d tan
in which d = depth buried, I length, and 6 = breadth of block,
< = angle of repose of soil and w = weight of soil per unit volume.
This rule does not appear to have a theoretical basis of any sort
and the fact that it gives reliable results in practice (if b is not less
than (say) 8 inches) is undoubtedly due to the neglect of two other
factors, viz. the COHESION of the soil and the INCLINATION or THE
PULL.
Consider a vertical retaining wall AB (Fig. 88) with horizontal
ground surface AC. It can be shown that if the wall is moved
horizontally towards the soil it retains, rupture takes place along
the line BC, making an angle of (45 + j ) with the vertical.
\ 2/
ANGLES AND TERMINALS
177
When, a stay block is placed at B in the usual way so as to bear
on undercut soil., the cohesion of the soil to the left of the vertical
AB is destroyed at least temporarily and may therefore be neglected.
If it be conceded that the line of rupture when the anchorage
fails may possibly be the line BC as denned above, then the vertical
rupture intensity of the soil ab the depth d will be due to the reaction
of the triangular mass of soil ABO. This reaction will be propor
tional to the weight of the mass, together with the cohesive force
tending to prevent separation along the line of rupture. Anything
like a complete expression for this reaction would be too cumber
some for practical use and, moreover, owing to the want of homo
geneity of the soil and the uncertainty of the values of the specific
weight, angle of repose, etc., it is fatuous to attempt close accuracy.
P
FIG. 88.
Suppose the stay block to be one foot wide, that the surface
is horizontal and the pull vertical. The weight of the volume of
soil of triangular section ABO and one foot in thickness, which con
tributes to hold down one foot lenth of the block
Ib.
in which w = weight of soil per cubic foot in pounds.
d = depth in feet.
(j> = angle of repose.
Now the cohesive force can be shown to be a function of the
height which the soil will stand when freshly cut, and also of the angle
of repose, but to simplify matters we will assume that the effect of
cohesion is apparently to increase the specific weight of the soil.
With the above assumptions, therefore, the vertical intensity of
178
OVERHEAD POWER LINES
pressure on tlie stay block when the anchorage is on the point oJ
giving way may "be written
p = Kwd*h tan (45 + ) Ib. / sq. ft.
FIG. 89.
h being the height in feet which the soil will stand when freshly cut
and K some constant yet tc
be determined.
Now consider the state o:
affairs when, as is usual, th(
pull is not vertical.
Let ab (Fig. 89) represenl
section of stay block of unii
area, the inclination of the pul
being cc with the grounc
surface.
Assume the ratio of th<
horizontal rupture intensity of the soil to the vertical ruptun
intensity to be (N + 1). Then if r equals the pull on the bloc!
at which it gives, we have
r . ab = p . ac . sin cc f (N f 1) . p . be . cos oc
= ^{sin 2 oc + (N 4. l) cos 2 oc}
= jj{l f N cos 2 oc} = p{l + N(l  sin 2 oc)}.
Substituting the value ioxp found above we get
45
 sin 2 oc)} Ib. / sq. ft.
It is realised that the above reasoning is open to criticism i]
many respects, but it is interesting to compare it with the following
very similar formula given in a paper in the Journal of the Institution
of Civil Eiu/ineers, in 1912, by Capt. (now Lt. Col.) C. E. P. Sankev
E.E. ret. :
r = Kwd z h sin 2^(1 f..#(l  sin oc)} Ib. / sq. ft.
It will be found that, providing suitable constants are choser
the results^ obtained with one formula are not very different fror.
those obtained with the other.
In the paper referred to, the experimentally determined value
for the constants K and N to suit the latter formula were given a
049 and 237 respectively, but admitting the difficulty of assignin
ANGLES AND TERMINALS
179
precise values to w, h and <j>, the following simplified expression
was suggested as being quite close enough for practical purposes.
As it has proved reliable for many years, its use is recommended.
r wd z h sin 20(15 sin oc) Ib. / sq. ft.
Although based on consideration of a block 1 foot broad, experi
ments show that the expression can be used for any breadth up to
several feet.
It has also been verified that the effective breadth of a round
log is equal to its diameter.
The holding power is reduced if the soil becomes wet. This can
be allowed for by reducing the value of /t.
Approximate values of the constants for various soils are given
in Table XVII.
TABLE XVII. Constants for Various Soils.
w,
Ib.
h,
feet.
*,
degrees.
Mud ....
90
Loose dry earth (loamy soil)
90
01
25
Ordinary surface earth (loamy s
jil)
90
13
25
Welldrained earth (loamy soil)
100
510
30
Moist earth (loamy soil)
100
13
40
Very wet earth (loamy soil)
100
01
15
Ordinary dry clay .
120
912
3035
Damp clay, well drained
120
48
45
Wot clay.
120
03
1520
Clean dry sand
100
01
3035
Wet sand.
100
15
2530
Clean gravel
110
01
4045
Damp shingle
100
.
40
Loam, with gravol
110
13
25
fcsand, with gravel
110
01
25
Clay, with gravel
110
13
3040
For well drained, loamy soil we may take as conservative values
w = 90, h = 5 and ^ = 30. Substituting these values in the for
mula and allowing a factor of safety of 25 as required by the E.G.
Regulations, we get
r = 156<Z a (l5 sin oc) Ib. / sq. ft.
Values of r for various depths and angles are plotted in Fig. 91.
In our example we will for simplicity deal with one stay anchorage
' only, assuming that it takes one third of the load.
180 OVERHEAD POWER LINES
It is advisable to have in all cases at least two distinct stays and
anchorages, separated by not less than 6 feet.
Case (1) Stay 15 feet from pole, T = il?? = 3 890 lb., oc = 60.
<j
8240
Case (2) Stay 26 feet from pole, T = ^ = 2 750 lb., oc = 45.
Assuming a depth of 5 feet, we find by reference to Fig. 91 that
the safe working load per square foot equals 3 100 lb. at 45 and
2 500 lb. at 60. Therefore the area of stay block required
n , . 3 890 , K _
Case(l) = =156 sq.ft.
Case (2) = ^ = 089 sq. ft.
Theoretically, if the stay block is 9 inches wide it need not
be longer than 208 feet in case
(1) and 119 feet in case (2).
X" Now consider the strength of
the block itself.
Strength of Stay Block
(Fig. 90). With a load of 3 890
lb. and a stay block 208 feet by 9 inches, what should be its
thickness?
First consider bending :
9 QQA
Distributed load per inch length =  = 156 lb.
a ,,72 1 KR 1 4R^
Maximum B.M. (at centre) =   = = 12 150 Ib.ins.
bd g
Moment of resistance of section of block = M = fZ = f .
J J Q
Q ,72
= 7 800 . ~ Ib.ins.
6
Allowing a factor of safety of 35
12 150 .35.6 _
~~ 7 800 . 9 ' ~
whence d 191 inches minmium.
ANGLES AND TERMINALS
181
Maximum shear stress
3890
113 Ib. /sq. in.
9 X 191. X 2
This is not excessive.
An iron washer 6 inches X 6 inches X mcn should always be
used under the head of the stay bolt, to distribute the pressure. This
washer is neglected in the above calculations and the values are
therefore on the safe side.
A stay block 24 inches X 9 inches X 2 inches thick will obviously
satisfy requirements, but as the cost of the stay blocks is a very small
percentage of the overall cost of the line there is no need to cut the
nooo
15 30 45_ 60 75 90 '
Inclination of Stay Wire with Ground (Deg.)
Fro. 91. Strength of anchorages in good soil.
dimensions too close and in practice blocks less than 3 feet long and
4 inches thick are seldom used.
It must be remembered that the stay block performs a very
important function and failure of an anchorage may result in the
destruction of a large number of supporting poles. It is out of sight
and usually out of mind, it may not be buried fully to the depth
prescribed, it may be damaged a little when fixing, the creosoting
may be imperfect, and the rate of decay more rapid than antici
pated, and, moreover, we can seldom fix precise values for h and <}4.
In conclusion, the use of thin sheetiron stay plates may be
referred to briefly.
182 OVERHEAD POWER LINES
Theoretically, tlie uniformly distributed breaking load o
iron plate 24 inches square, if assumed to be supported at the ce
only by a 6 inch X 6 inch washer under the bolt head, is a
5 000 Ib. for a plate & inch thick and 9 000 Ib. for a plate
thick.
Actually plates will support greater loads than these, since
bending which occurs causes a redistribution of the load as ii
creases, with a maximum value at the centre, and further, in
initial stages, the reaction of the earth below the plates opj
the tendency to bend.
Experimentally, a plate 24 inches X 24 inches X fV inch
found to stand up to 14 000 to 16 000 Ib. temporarily and gsw
9 000 to 11 000 Ib. when sustained. Figures for a 24 inch >
inch. X i inch plate are 20 000 and 15 000 approximately.
It may be inferred from this that the safe maximum wor
load for a 24 inch X 24 inch plate is 5^ = 4 000 Ib. when ^
i'D
thick and ' r == 6 000 Ib. when 1 inch thick.
25 *
To take full advantage of the bearing surface and use the va
given in Fig. 91, it will clearly be necessary to reinforce the pi
radially.
183
CHAPTER X.
CONDUCTORS OTHER THAN COPPER.
CONSIDERATION of other conductor materials has been purposely
postponed, because comparison between the various more or less
suitable materials is very much influenced by their mechanical
properties as well as their electrical.
The main desiderata in a conductor material are :
1 . Low price.
2. High specific electrical conductivity.
3. High tensile strength.
4. Chemical inertness to atmospheric effects.
5. Ease of erection and jointing.
Particulars of various conductor materials are given in Table
XVIII.
Pure harddrawn copper most closely satisfies all the required
conditions. It is still the best known and most widely used material
and at its present price it is likely to remain so both for low voltage
and high voltage distribution. It lasts indefinitely in use under
ordinary atmospheric conditions and it has a high scrap value.
Copper Alloys. Owing, however, to the large ratio of sag to
span length necessary with the smaller copper wires for high voltage
lines, a conductor of greater tensile strength is desirable for the
transmission of small amounts of power.
Bronze. The addition of a little tin and silicon to copper pro
duces an alloy known as " bronze " which has a much greater tensile
strength than copper, but unfortunately the gain in strength is
only obtained at the expense of a reduction in conductivity. The
standard P.O. silicon bronze (B.S. Spec. 1751923) has a tensile
strength of about 100 000 Ib. / sq. in., which is a 70 % increase on
that of H.D. copper, but its conductivity is reduced by more than
100 %. For equal conductivity it costs more than twice as much
as pure copper.
184
OVERHEAD POWER LINES
8
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Density of
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CONDUCTORS OTHER THAN COPPER 185
CopperCadmium. A coppercadmium alloy which, has
recently been placed on the market seems to be much more
promising than bronze, a 35 % increase in strength being secured at
the expense of 16 % only in conductivity. For equal conductivity
it costs about 30 % more than pure copper (see Table XVIII.).
CopperSteel. Since high tensile steel can be obtained with
a breaking stress exceeding 200 000 Ib. / sq. in. compared with 60 000
Ib. / sq. in. lor H.D. copper, it is natural that endeavours should
have been made to combine the strength of the former with the
conductivity of the latter. A method which suggests itself is to
lay up an annular layer of copper wire strands around a steel core,
but the objection to this is that the galvanising would quickly
disappear by electrochemical action as it is in' contact with copper.
This method is used very largely to reinforce aluminium conductors
and will be again referred to later.
In the U.S.A. a copperclad steel conductor has been used for
many years 3 the copper coating being metallurgically welded on to
the steel core, rendering the latter absolutely immune from corrosion.
Aluminium. The only other metal in competition with copper
from a purely electrical point of view is aluminium. At the prices
now ruling, aluminium and copper cost about the same for equal
conductivity.
It may be said at once, however, that aluminium is not a serious
competitor for light overhead lines, owing to the following disad
vantages :
(1) Lower Tensile Strength. This necessitates a larger ratio of
sag to span length, which in turn means longer and stronger
poles. (As a partial setoff, however, the terminal stresses
are not so great.)
(2) Greater Diameter. This increases the lateral wind loading
and therefore somewhat stronger supports are required
for this reason as well.
(3) Greater Ratio of Wind Load, to Weight of Wire. This neces
sitates greater clearances, as the wires are deflected more
from the vertical than copper wires of the same conduc
tivity. This fact often rules out the smaller aluminium
wires altogether.
(4) More care is required in erecting and jointing.
(5) Smaller scrap value.
The first three disadvantages can be studied by reference to
Tables XIX. and XX.
186
OVERHEAD POWER LINES
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188 OVERHEAD POWER LINES
SteelCored Aluminium. To overcome the first disadvantage
of ahuninium noted above, steelcored conductors are used, i.e.
conductors made up in the usual way with concentric layers of
aluminium wires, but in which the central wire or wires are of high
tensile steel. The objection to using a steel core in stranded copper
conductors, referred to on page 185, does not appear to hold with
aluminium. The explanation is said to be found in the tight bedding
of the relatively soft aluminium wires around the core, which,
combined with the final filling of any small interstices between the
outer aluminium wires with oxide, prevents any penetration of mois
ture inside the conductor. In addition, the fact that zinc and
aluminium have a very small electrochemical potential difference
undoubtedly has a bearing on the immunity from corrosion.
Such a composite conductor is considerably stronger than the
electrically equivalent H.D. copper conductor (see Tables XIX. and
XX.), therefore the ratio of sag to span length is smaller. This
means that longer spans can be used with steelcored aluminium
than with copper and this is of great importance in the transmission
of large amounts of power over long distances at very high pressures,
which is oubside the scope of this work. It is being used extensively
on the 132 000 volt main transmission lines now being erected in
this country.
It will be noted in Table XX that steelcored aluminium. shows a
saving of 4 to 5 % on the cost of supports and conductors, which
means about 2 % on the overall cost of the line. Considering its
disadvantages it is not a very attractive proposition for the con
ditions assumed, but it shows far greater economy in long span
work.
For distribution purposes where comparatively short distances
are involved, a very appreciable saving in first cost must be realised
to outweigh the difficulties due to jointing and branch connections.
This opinion is based on experience with aluminium lines in this
country in the last fifteen years, during which, compared with
copper, it has not shown up very well. The main difficulty experi
enced has been in maintaining continuity with parallelgroove clamp
connections, the contact surfaces of which sooner or later become
oxidised, however tightly they are clamped together.
However, the aluminium obtainable today is much superior in
purity to most of that on which the ab.ove opinion is based, and
joints undoubtedly give less trouble with purer metal. Also the use
CONDUCTORS OTHER THAN COPPER
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190
OVERHEAD POWER LINES
TABLK XXII. Sags and Tensions. Steel Conductors (45 ton Quality)
for .Erection Purposes. High Voltage Lines (%inch ice).
300
7/16 S.W.G. (064).
7/12 S.W.G. (104).
7/8 S.W.G. (16).
J
1
Ji'ahfc.
RflR,
Tension,
Sag,
Tension,
Sag,
Tension,
loot.
Ib.
feel;.
Ib.
foot.
Ib.
122
152
257
78
1320
05
3700
82
104
375
57
1 810
51
4790
02
88
400
50
2000
40
5310
42
74
527
45
2 290
42
5 820
22
04
010
41
2510
38
0430
22
332
1080
156
2820
92
6 GOO
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122
558
157
189
1 230
152
3020
82
485
181
144
1 010
119
4020
02
450
195
128
1810
107
5140
42
410
214
115
2 020
98
5010
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309
238
104
2 400
90
0110
22
14.11
1080
351
2820
207
6600
)
122
1140
137
405
1020
280
3 500
82
1080
144
319
1290
224
4 370
02
1045
149
283
1400
202
4850
42
1008
155
252
1040
184
5310
22
971
100
220
1830
108
5820
22
1328
1080
624
2820
368
6600
Basic loading sags and maximum legal tensions shown in italics.
of " cone " types oi connections wherever possible instead of a
clamp will reduce the jointing troubles.
Consequently, if there should be an appreciable fall in the price
of aluminium compared with that of copper, the above views may
have to be modified.
Galvanised Steel. In the above consideration steel has been
used really as a carrier for the conductor, but in cases where the
BI^O of copper or copperalloy conductor required from a purely
electrical point of view is too small for mechanical reasons it will be
sometimes quite feasible to use galvanised steel as the conductor
itHolf, in spite of its high resistance. The inductance is naturally
greater than that of nonmagnetic conductors, but for the small
ciirrerrts with which steel is likely to be used the ohmic resistance will
CONDUCTORS OTHER THAN COPPER
191
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192 OVERHEAD POWER LINES
usually be the predominating factor (see Table XXI). Skin effect
is inappreciable.
But unfortunately the life of steel is comparatively short and
its scrap value is negligible. It quickly rusts when exposed to the
atmosphere and must therefore be well galvanised for O.H. line work.
The tensile strength of steel increases with the carbon content,
but the cost and the specific resistance go up as well ; moreover
as the strength increases the steel becomes less ductile and flexible
and it rusfcs more quickly. High tensile steel having a breaking
stress of 180 000 to 200 000 Ib. / sq. in. is used in the cores of alu
miniumsteel cables and is reputed to be immune from corrosion
(see p. 188). But it is not usual to employ steel of greater strength
than 100 000 Ib. / sq. in. for O.H. powerline conductors, and this
will have a shorter life than the standard wires of lower tensile
strength used by the post office for telegraph line conductors and
stays (B.S. Specs. 182 and 1831923). The latter last upwards
of 20 years, except in certain manufacturing areas, whereas the
former will probably not last 15 years.
Particulars of three sizes of steel conductor are given in Table
XXI. and sag and tension values for erection purposes in Table
XXII. Table XXIII. illustrates the economy due to the use of these
conductors.
193
CHAPTER XI.
SAFETY PEECAUTIONS.
THE legal regulations on the subject are prescribed by the Electricity
Commissioners and the PostmasterGeneral and they are given in
full in Appendices I. to IV.
Provisions to Prevent Danger from
1. LEAKAGE.
A. Low AND MEDIUM VOLTAGE (Eegulations 13). For pres
sures to earth above 250 volts D.C. or 125 volts A.C.
(i) METAL POLES. A continuous earthed wire must be pro
vided, running from pole to pole and connected to all
the poles. It must be emphasised that an iron pole
does not make a very good earth contact in itself,
particularly when, as is frequently the case, it is set
in concrete. Hence the danger of touching a pole on
which there is a faulty insulator, if a good earth con
nection is not provided.
(ii) WOOD POLES. In cases where an earth wire does not
form part of the conducting system, all ironwork should
be bonded to the earth wire. In cases where there are
no earth wires, the ironwork should be bonded together
to allow a greater surface for the dissipation of leakage
currents. Where the leakage on a line is considerable
and the resistance of the earth connections is high, it
is possible for the potential of the bonding wires to be
raised to a dangerous value. For this reason, if bonding
wires or lightning conductors are led down a pole to
earth plates it is necessary to insulate the bonding
wires to a height of 10 feet from the ground. A cover
ing of creosoted wood casing suffices.
STAY WIRES on wood, poles must be considered as
13
194 OVERHEAD POWER LINES
part of the metal work and an insulator must be placed
in each stay wire at a height of not less than 10 feet
from the ground.
It should be noted that a neutral conductor must
be earthed at one point only, viz. the generating station
or substation, and it cannot, therefore, be considered
as a continuous earthed wire for purposes of complying
with this regulation (but see page 54).
SERVICE LINES. Special attention is drawn to
Regulation 8, which lays down that service lines where
accessible must be insulated. To comply with this
regulation, all conductors should be covered with
durable insulating material within 6 feet of a building.
Many fatal accidents have occurred due to neglect of
this regulation.
B. HIGH VOLTAGE LINES (Regulation 16). In the case of
H.V. lines the danger from leakage is obviously greater
and all metal work other than the conductor must be
bonded together and connected with earth in all cases.
Tinned copper or galvanised iron bonding wire should
be used. The earthing may be carried out by a con
tinuous wire, earthed 4 times per mile, or, alternatively,
the continuous wire may be omitted and the metal
work effectively earthed at each pole.
The former method is nearly always adopted, since
effective earthing is nqt generally practicable at every
pole, and although the continuous wire adds consider
ably to the cost of the line it affords a measure of pro
tection against atmospheric effects and it is also useful
as a support for auxiliary conductors or telephone
cables.
The earth connecting wire should be enclosed in
creosote wood casing for a distance of 10 feet from
ground level.
2. BEOKEN LINE CONDUCTOE.
A. Low AND MEDIUM VOLTAGE. The possibility of a con
ductor falling when erected in accordance with the E.G.
Regulations must be admitted to be remote, but it has
to be considered. E.G. Regulation 13 stipulates the
provision of a continuous neutral or other earthed wire
SAFETY PRECAUTIONS
carried from pole to pole and so arranged as to m
contact with a falling conductor.
Most L.V. distribution systems work with eart'
neutral conductor, which can fortunately be used a
continuous earthed wire to comply with this regulati
It was formerly the practice to use a soca]
" split " neutral conductor as illustrated in Figs,
and 34 and fix two cross wires in each span as far
from the pole as the lineman could reach or alter
tively, a single neutral conductor sufficed if a triangu
guard was fitted as shown in Kg. 36.
But providing the neutral conductor is plai
directly below the other conductors, the triangi
guard may now be dispensed with, as in Fig. 35.
When the line conductors are arranged in triangi
fashion as in Fig. 33 and 3d one earthed conductor i
now suffice if it is staggered from side to side at e;
succeeding support. As stated before, this methoc
construction appears to the writer to present difficult
and the vertical arrangement of Figs. 35 and 36 is r
likely to be the most popular.
B. HIGH VOLTAGE. No special guarding against brol
conductors is required, on H.V. lines, except in
neighbourhood of roads, railways, etc., but attent
may be drawn to the following clause in Eegulation '.
" The design and construction of the system of ea
connections shall be such that when contact is rcu
between a line conductor and metal connected w
earth, the resulting leakage current shall not be 1
than twice the leakage current required to operate
devices which make the line dead."
This implies that ordinary overload protection
not sensitive enough for H.V. lines unless three c
ditions are satisfied, viz., (i) the neutral of the systen
earthed, (ii) suitable guards or earth bars are fit
throughout the line so that a broken conductor ^
make contact with earth, and (iii) the overload ti
act instantaneously (i.e. no time lag attachments
permitted).
There are available a number of very sensii
19 6 OVERHEAD POWER LINES
protective devices, with wliicli a line conductor on
"breaking is made " dead " in a very small fraction of
a second, before it has time to reach a person under
neath. But although admittedly the best forms of
protection, they entail special line construction with
auxiliary conductors and the cost is generally pro
hibitive for minor H.V. distribution lines. For such
lines ordinary " leakage " protection will suffice. With
earthed neutral at the generating station or transformer
station, leakage relays can be relied upon to cut off a
line in onetenth of a second when a leakage current
of the order of 5 % of the normal full load current
flows, although it may not often be necessary or de
sirable to set the relays too lightly.
Additional Safety Precautions in the Neighbourhood
of Roads, Railway and Canals.
A. L.V. AND M.V. LINES. The regulations for these lines
are framed on the assumption that they will be used
in residential areas and therefore always in the neigh
bourhood of roads, etc., and except for increased
clearances from ground no further precautions are
laid down.
B. H.V. LINES.
(i) CLBAEANGE PROM GROUND. In ordinary cross country
work, the specified ground clearance of 20 feet is meas
ured from the lowest line conductor, but when the
line is erected along or across a public road the clear
ance must be taken from the lowest wire on the pole.
This means that the poles must be several feet longer
near public roads if earth wires or auxiliary conductors
are employed.
(ii) WITHIN 50 FEET OP A EOAD OR CANAL. The following
alternative safety devices are prescribed :
either (a) Duplicate insulators and strap wire ;
or (6) Single insulators plus an earthing device.
Ref. (a) The duplicate insulators are arranged as
shown in Tigs. 26 and 32, and a strap wire of HJD.
copper of the same size as the conductor, or, alter
natively, of phosphor bronze of the same strength con
nects the second insulator to points on the conductor
SAFETY PRECAUTIONS 197
as far out from the pole on either side as the lineman
can reach, i.e. from 3 to 4 feet. This strap wire is
intended to prevent the wire from falling to the ground
in the event of the line conductor being burnt through
at a faulty insulator.
That is to say, it is the insulator primarily which
causes the anxiety.
Ref. (6) An earthing bar or bow of galvanised iron
or copper fixed under each conductor is commonly
used as shown in Figs. 32 and 37, but the triangular
guard arrangement shown in Fig. 36 for L.V. lines is
quite suitable on H.V. lines if the conductors are
arranged in a vertical plane. The triangular guard
can be secured to the earth wire at points 4 feet from
the pole and from this point of view it is really better
than an earthing bar, which as generally fitted does
not usually extend outwards from the pole for more
than 18 inches or 2 feet.
(iii) CROSSING A ROAD, CANAL OR RAILWAY. The following
methods of construction comply with the regulations :
Either (a) Duplicate insulators plus duplicate con
ductors ;
or (b) Duplicate insulators and strap wire plus
earthing device.
It will be noted that the regulations make no reservation as to
length of span or angle of crossing. It was formerly the practice to
shorten up the span at a road crossing as it was believed that a
greater factor of safety could be thereby ensured, but it is now gene
rally agreed that it is not good practice to do this. Of 'the two
alternative methods, the second is by far the better, as it does not
add to the line loading in any way. Among the objections to the
first method are the necessary stronger poles with additional line
stays, and tensioning insulators on the second conductors.
Stranded conductors are preferable to solid ones in the neigh
bourhood of roads.
With regard to railway crossings, the railway authorities do not
accept the E.G. regulations without qualification and they should
always be consulted when surveying a route which crosses their
permanent lines. They usually require duplicate conductors with
the power line at right angles to the rails and span as short as possible.
198 OVERHEAD POWER LINES
Post Office Regulations. In addition to those prescribed
by the Electricity Commissioners further regulations laid down by
the PostmasterGeneral must be complied with when power lines
are erected in the neighbourhood of signal wires. The P.O. Engin
eering Department's Memoranda on the subject (T.E. 80 and E. in
C. 231) are given in full in Appendices III. and IV., but attention is
drawn to the following points.
A. Low AND MEDIUM VOLTAGE.
GUARD WIRES AT CROSSINGS. When practicable the crossing
should always be effected with the power conductors above, as being
invariably larger wires, erected with a higher factor of safety, they
are less likely to fall than the signal wires, and, moreover, it will not
be necessary to interrupt the power supply when signal wires are
being repaired.
Further, the crossing should preferably be as nearly at right
angles as possible, as the requirements are then simpler.
Independent guard wires may be used in all cases, but if the
pressure to earth does not exceed 250 A.C. and the signal wires are
underneath, it will usually be found more convenient and economical
to split the neutral of the power system at the crossing span, cross
lacing every 6 feet as required.
If the signal wires are above, it will be necessary to run an ad
ditional earthed wire above the power wires, unless the arrangement
shown in Tig. 34 is adopted with split neutral and cross lacing.
The normal specified clearance between guard wires and signal
wires is 4 feet, but in special cases a clearance of 2 feet may be ac
cepted.
It will be noted that no guard wires are required when the power
conductors are insulated and supported by an earthed bare suspen
sion wire, nor are they required up to 250 volts A.C. if either the power
conductors or the signal wires are insulated with an approved weather
proof covering (see Specification, p. 232).
HIGH VOLTAGE. The P.O. still prefer (but do not now insist)
that either the power wires or the signal wires should be placed
underground at crossings.
The requirements are roughly indicated in Eig. 92.
In special cases, however, the department has agreed to smaller
values of x and a', but this has never been less than 1 G f , the height
of the signal route, or 10, the height of the power circuit route,
whichever is the greater. This allows a smaller clearance than the
SAFETY PRECAUTIONS
199
standard rule in cases where the power wires are higher than the
signal wires.
But apart from the question of cost there are important electrical
objections to inserting short lengths of cable in either line. This is
now more fully realised than formerly and the problem became acute
with the advent of 33 000 volt and 66 000 volt transmission.
The latest P.O. regulations, therefore, permit overhead crossings
with duplicate conductors plus a cradle guard, which is a compromise
to b
orC'^Ju'cherei* is the qreoter) + d (difference of fetv/J
FIG. 92.
that may cost as much as a cable crossing, but is more satisfactory
from an electrical point of view.
It may also be noted that consent has been obtained to suspend
an armoured cable from a steel wire, which costs much less than
putting it underground, because the use of terminal poles and stays
is avoided, the line tension being carried through by the suspension
In this connection it may be noted that up to 6 600 volts, wire
armoured, ozoneproof cable is better than leadcovered paper cable,
as it is lighter and the cost of sealing boxes is avoided.
301
APPENDIX I.
ELECTBICITY (SUPPLY) ACTS, 1882 TO 1926. [El. C. 53.]
OVERHEAD LINE REGULATIONS for securing the Safety of the Public
made by the Electricity Commissioners under the Electricity
(Supply] Acts, 1882 to 1926.
DEFINITION.
In the following Regulations the expression " line conductors "
means conductors used for transmitting a supply of electrical energy,
including so much of any service line as may be under the control of
the undertakers.
I. GENEBAL.
Material of Line Conductors.
1 . Line conductors shall be of copper, aluminium, or such other
materials as may be approved by the Electricity Commissioners.
Strength of Line Conductors.
2. All line conductors at the time of erection shall comply, as
regards elongation, breaking load and elasticity, with the specifica
tion of the British Engineering Standards Association then in force.
Minimum Size of Line Conductor.
3. The minimum permissible size for copper and other line
conductors (other than service lines) shall be such as to have an
actual breaking load of not less than 1237 pounds, the equivalent
minimum crosssectional area and weight per mile for copper being
as follows :
Conductor. Crosssectional area, Weight per Mile,
sq.. ins. Ibs.
No. SS.W.a . 00201 409
202 OVERHEAD POWER LINES
The minimum permissible size of service line shall be such, as to
have an actual breaking load of not less than 816 pounds, the equiva
lent minimum crosssectional area and weight per mile for copper
being as follows :
Conductor. Crosssectional area, Weight per Mile,
sq. ins. Ibs.
No. 10 S.W.G. 00129 262
Line Conductors to be inaccessible.
4. Line conductors shall be rendered inaccessible to any person
from any building or other place without the use of a ladder or other
special appliance.
Regard shall be had to the normal use by the occupier of any
premises or land and where necessary (a) the height of the line
conductors shall be increased to provide sufficient clearance for
safety in accordance with such use, and (6) provision as hereinafter
prescribed in Regulations 14 or 17 shall be made to prevent danger.
Line Conductors Crossing other Lines.
5. "Where a line conductor crosses over or under, or is in proximity
to any other overhead wire, precautions shall be taken by the
undertakers to prevent contact, due to breakage or otherwise,
between the line conductor and the other overhead wire, or between
the other wire and the line conductor.
Provided that this Regulation shall not be deemed to require
the undertakers to take precautions against contact between a
broken line conductor and other auxiliary conductors and earth
wires carried on the same support and forming part of the same
overhead line.
Supports.
6. Line conductors shall be attached to suitable insulators carried
on supports of wood, iron, steel or reinforced concrete. All wooden
supports other than oak or hard wood crossarms shall unless other
wise approved by the Electricity Commissioners be of red fir impreg
nated with creosote. Special precautions shall be taken to prevent
the corrosion of all metal work at or below the surface of the ground.
Factor of Safety of Supports.
7. The supports, in conjunction with stays or struts if provided,
shah 1 withstand the longitudinal, transverse and vertical loads due
*" ^e ice loadings and wind pressure hereinafter specified without
ge and without movement in the ground. In no case shall
APPENDIX I 203
the strength of a support in the direction of the overhead line be less
than onequarter the required strength in a direction transverse to
the line.
The following factors of safety shall apply to each support :
Material. Factor of Safety.
Iron or steel . . . 25
Wood .... 35
Reinforced concrete . . 35
These factors of safety shall be calculated on the assumption
that all line conductors cables and wires carried by the supports
are at a temperature of 22 E., and have a covering of ice to the radial
thickness specified in Regulation 12 (or Regulation 15 according to
the voltage) and that together with the supports they are subjected
to a, wind of 50 miles per hour at right angles to the line, this wind
to be taken as exerting a pressure equivalent to 8 pounds per square
foot calculated on the whole of the projected area.
The wind pressure on the lee side members of lattice steel or
other compound structures, including A and H poles, shall be taken
as onehalf of the wind pressure on the windward side members.
The factor of safety shall be calculated on the crippling load of struts
and upon the elastic limit of tension members.
Service Lines.
8. Service lines shall be connected to line conductors at a point
of support only and shall be fixed to insulators on consumers'
premises. Every part of a service line (other than a neutral con
ductor connected with earth) which is accessible from a building
with the use of a ladder or other special appliance shall be efficiently
protected either by insulating material or by other means approved
by the Electricity Commissioners.
Erection of Line Conductors at Different Voltages on same Supports.
9. Where line conductors forming parts of systems at different
voltages are erected on the same poles or supports adequate pro
vision shall be made to guard against danger to linesmen and from
the lower voltage system being charged above its normal voltage by
leakage from or contact with the higher voltage system ; and the
type of construction shall be subject to the prior approval of the
Electricity Commissioners.
204 OVERHEAD POWER LINES
Inspection and Maintenance of Lines.
10. Every overhead line, including its supports and structural
parts, and electrical appliances and devices belonging to or con
nected therewith, shall be regularly inspected and efficiently main
tained.
Materials used,
11. All materials used shall at the time of erection conform to
the specifications of the British Engineering Standards Association
and to the Post Office Technical Instructions for the construction
of aerial lines for the time being in force, so far as the same are
applicable and are not inconsistent with these Regulations.
II. SPECIFIC REGULATIONS.
[Applicable according to the voltage between line conductors where
no part of the system is connected with earth, or according to the voltage
to earth where part of the system is connected with earth,]
A. For voltages not exceeding 650 volts direct current and 325
volts alternating current.
Factor of Safety of Line Conductors.
12. The factor of safety of line conductors shall be 2. The
factor of safety shall be based on the breaking load and shall be
calculated on the assumption that the line conductors are at a tem
perature of 22 F. and have a covering of ice to a radial thickness of
threesixteenths of an inch, and that they are simultaneously subjected
to a wind of 50 miles per hour at right angles to the line, this wind
to be taken as exerting a pressure equivalent to 8 pounds per square
foot calculated on the whole of the projected area of the icecovered
lines.
The weight of ice is to be taken as 57 pounds per cubic foot.
The elasticity of the metal may be allowed for in calculating the
sag for line conductors.
Minimum Height of Conductors.
13. The height from the ground of any line conductor (other
than a service line), earth wire, or auxiliary conductor at any point
of the span at a temperature of 122 E. shall not, except with the
consent of the Electricity Commissioners, be less than 19 feet across
a public road or 17 feet in other positions. A height of 15 feet may
be adopted in situations inaccessible to vehicular traffic.
APPENDIX I
Where a service line is carried across or along a carriag*
the height of the line from the ground at any part of the cai
way shall not, except with the consent of the Electricity Co:
sioners, be less than 19 feet and 17 feet respectively.
Provision to Prevent Danger.
14. Where the voltage to earth exceeds 250 volts direct ci
or 125 volts alternating current, precautions should be tat
prevent danger
(1) from a broken line conductor by the provision of
(a) a neutral or earthed conductor carried contini
from pole to pole, and so arranged in relation
other conductors that in the event of breakage <
one of them the line conductor shall make c<
with the earthed wire ; or
(6) other means approved by the Electricity Commissi
(2) from leakage by the provision
(a) in cases where metal poles are used, of
(i) an earthed wire, running from pole to pole an<
nected to the poles ; or
(ii) a suitable metal framework to support the insi
carrying the line conductors, the framework
insulated from the pole but connected to the n
conductor ; or
(iii) other means approved by the Electricity Co
sioners.
(6) in cases where wooden poles are used, of
(i) a bonding wire connected to the supporting :
work of all insulators, the bonding wire termi:
at the lowest part of the supporting metal
or
(ii) other means approved by the Electricity Co
sioners.
Where lightning conductors are used or other uninsulate<
ductors are run down wooden poles to within 10 feet fro
ground, the precautions for the prevention of danger from 1<
shall be as for metal poles.
All stay wires other than those which are connected witl
by means of a continuous earth wire shall be insulated to p
danger from leakage. For this purpose an insulator shall be
in each stay wire at a height of not less than 10 feet from the g
B. For voltages exceeding 650 volts direct current and 32
alternating current.
206 OVERHEAD POWER LINES
Factor of Safety of Line Conductors.
15. The factor of safety of line conductors snail be 2. The factor
of safety shall be based on the breaking load and shall be calculated
on the assumption that the line conductors are at a temperature of
22 E., and have a covering of ice to a radial thickness of three
eiglths of an inch, and that they are simultaneously subjected to a
wind of 50 miles per hour at right angles to the line, this wind to be
taken as exerting a pressure equivalent to 8 pounds per square foot
calculated on the whole of the projected area of the icecovered lines.
The weight of ice is to be taken as 57 pounds per cubic foot.
The elasticity of the metal may be allowed for in calculating the
sag for line conductors.
Minimum Height of Gondiictors.
16. The height from the ground of any line conductor at any
point on the span at a temperature of 122 F. shall not, except with
the consent of the Electricity Commissioners, be less than the height
hereunder stated :
Voltages not exceeding\ 9n , , Voltages exceeding"]
66 000 volts. ' / U 110 000 volts and not I ^ f
Voltages exceeding^ exceeding 165 000 j
66 000 volts and not 1 9 , , , volts. J
exceeding 110 000 I ^ el " Voltages exceeding^ ,
volts. ^ J 165 000 volts. j^teet.
The height from the ground of an earth, wire or auxiliary con
ductor shall not be less than the minimum heights prescribed in
Regulation 13.
Provision to Prevent Danger.
17. Adequate means shall be provided to render any line con
ductor dead in the event of it falling due to breakage or otherwise.
All metal work other than conductors shall be permanently and
efficiently connected with earth. Eor this purpose a continuous earth,
wire shall be provided and connected with, earth at four points in
every mile, the spacing between the points being as nearly equidis
tant as possible, or, alternatively, the metal work shall be connected
to an effective earthing device at each individual support. The
design and construction of the system of earth connections shall be
such that when contact is made between, a line conductor and metal
connected with earth the resulting leakage current shall not be less
than twice the leakage current required to operate the devices which,
make the line dead.
APPENDIX I
Road Crossings, cfec.
18. Where an overhead line is erected along or across a pub
road or canal or across a railway all wires including earth, wires ai
auxiliary conductors shall be placed at the appropriate height frc
the ground specified in Regulation 16 for line conductors, and t
following additional precautions shall be taken to prevent danger :
(1) In the case of a line erected along a public road or canal (
within 50 feet thereof) there shall be provided
(a) duplicate insulators supporting the conductors ; or
(6) a device to ensure that in the event of a line conduct
falling it shall be put to earth ; or
(c) other means approved by the Electricity Commissione:
(2) In the case of a line erected across a public road, canal
railway there shall be provided
(a) duplicate insulators for supporting the line conductor a
a device to ensure that in the event of a line cc
ductor falling it shall be put to earth ; or
(b) duplicate insulators supporting duplicate conductors ti
at intervals not exceeding five feet ; or
(c) other means approved by the Electricity Commissione
Danger Notices.
19. Supports shall be numbered consecutively and each suppi
shall have a danger notice of a permanent character securely 'fix
to it. Adequate provision shall also be made to prevent unauth<
ised climbing.
These Regulations are made subject to the power of the EL
tricity Commissioners to make such further or other Regulatic
as they may think expedient and shall apply to any overhead Hi
erected by authorised undertakers :
Provided that these [Regulations shall not apply to any overhe
lines in existence at the date hereof and constructed and maintain
by authorised undertakers under and in accordance with the p:
visions of any prior Regulations for overhead lines made by 1
Board of Trade or the Electricity Commissioners.
Signed by order of the Electricity Commissioners this 16th d
of April, 1928.
R. T, Gk B 1 BENCH,
Secretary to the Electricity Commission*
208 OVERHEAD POWER LINES
ELECTRICITY COMMISSION.
EXPLANATORY MEMORANDUM on the Revised Code of Overhead Line
Regulations (El. C. 53A) made by the Electricity Commissioners
and on matters relevant thereto.
The Electricity Commissioners have recently reviewed the Code
of Overhead Line Regulations (El. C. 39) adopted by them in the
latfcer part of 1923, with the object of determining whether relaxa
tions consistent with the maintenance of a reasonable degree of
safety to the public could be made for the purpose more particularly
of facilitating the development of overhead distribution at the lower
voltages in rural areas.
Due regard has been given to various representations on this
matter which have been made to the Commissioners from time to
time on behalf of the Electricity Supply Industry, and the technical
aspects have formed the subject of conferences between the Com
missioners and Electrical Engineers who have had considerable
experience in the erection of overhead lines in rural districts and
elsewhere. The Commissioners have also conferred with the Post
masterGeneral on the question of the relaxation of his requirements
as to protection between telegraph or telephone lines and electric
supply lines.
As a result, the Commissioners have adopted a revised Code of
Overhead Line Regulations (EL C. 53) with effect as from 16th
April, 1928, and the following explanatory notes dealing with the
various relaxations and with other relevant questions have been
prepared for the information of the Electricity Supply Industry.
REGULATION 1 (Materials for Line Conductors).
This Regulation remains unaltered, but it may be noted that the
" other materials " for line conductors which have already been
approved by the Electricity Commissioners in certain cases include
steelcored aluminium, steel, cadmium copper and other appropriate
alloys of high tensile strength.
REGULATION 3 (Minimum Size of Line Conductor).
In view of the relaxation which has been made in the assumed
ice loading on line conductors (Regulations 12 and 15) and in guard
ing requirements (Regulation 14), the minimum size of copper con
ductor (other than service lines) is in future to be No. 8 S.W.G. or the
nearest equivalent section of stranded conductor (being not less than.
APPENDIX I 209
No. 8 S.W.G.). The minimum size of lines in general (other than
service lines) is to be such that the actual breaking load shall be not
less than 1 237 Ib.
REGULATION 6 (Supports}.
Under this Regulation, as amended, the Commissioners will be
prepared in special cases to consent to the use of wooden poles other
than of red fir, provided that suitable precautions are taken in the
felling, selection and treatment of the timber.
REGULATION 7 (Factor of Safety of Supports).
After consultation with the Air Ministry, the National Physical
Laboratory and the British Electrical and Allied Industries Research
Association on the questions of climatic conditions, wind velocities
and wind pressures, the Commissioners have decided to retain the
same wind pressure and the same factors of safety as are prescribed
'by the existing Code in respect of supports and also of line conductors.
The wind pressure on the supports alone has been shown to vary with
different types of support, but is usually only a small proportion of
the total load on the supports.
The Commissioners have concluded, however, that the relaxed
conditions with regard to the assumed ice loading on line conductors
(Regulations 12 and 15) will enable supports of suitable size and
strength, but of less expensive construction than hitherto, to be
adopted consistently with securing the safety of the public. An
enquiry is being continued into the factor of safety of reinforced
concrete supports.
With regard to the foundation of supports, the Regulation now
requires that the supports shall withstand the specified ice loadings
and wind pressure without movement in the ground. As the working
load under the Revised Code will constitute a greater proportion of
the ultimate strength of the supports than under the prior Code, it
will be more particularly necessary to pay attention to the founda
tions of wooden poles, which will require the addition of cross mem
bers or " kicking blocks " (excepting in the case of poles of small
sizes) in order that the foundation may be as strong as the pole itself.
REGULATION 8 (Service Lines}.
The requirement for the protection of persons working on the
outside of buildings may be generally assumed to involve the cover
ing of all live conductors with durable insulating material for so
much of their length as lies within 6 feet of the building.
14
210 OVERHEAD POWER LINES
REGULATION 9 (Erection of line Conductors at Different Voltages on
same Supports}.
Although the construction of lines at different voltages on the
same supports was not precluded under the prior Code, the Com
missioners have considered it desirable to include a new Regulation
dealing specifically with this matter. The Commissioners will be
prepared to approve of types of construction under this Regulation
which make adequate provision for avoiding danger to linesmen
working on the lines and for preventing contact between the higher
and lower voltage systems.
REGULATION 12 (Factor of Safety of Line Conductors}.
The Commissioners have made a relaxation in the assumed ice
loading on lower voltage lines from onequarter of an inch to three
sixteenths of an inch, but, as previously indicated, have retained the
provisions of the prior Code as to wind pressure and factor of safety.
The assumed ice loading and unaltered factor of safety will permit
of line conductors being strung to a tension which is at least as high
as may profitably be used and is consistent with the tension when the
line withoutthe assumed ice loading is subjected to a wind of gale
force equivalent to a pressure of 20 Ib. per sq. foot.
REGULATION 13 (Minimum Height of Conductors}.
The minimum height from the ground of lower voltage line con
ductors has been reduced from 20 feet to 19 feet across public roads ;
to 17 feet along public roads and in other positions ; and to 15 feet
in situations inaccessible to vehicular traffic.
The minunum height of service lines has been reduced from 20
feet to 19 feet across carriageways and to 17 feet along carriage
ways.
REGULATION 14 (Provision to Prevent Danger}.
Provided that one of the conductors is properly connected to
earth at the point of supply and that the lines are so arranged that
the earthed conductor is placed below the other conductors, no other
guarding on lower voltage lines will be required.
If the conductors are arranged in a vertical plane with the earthed
conductor placed lowermost, the protection afforded is adequate.
If the conductors are not so arranged, then the earthed conductor,
which must still be erected in the lowermost position, should be
staggered from right to left of each succeeding support so as to afford
APPENDIX I
a reasonable certainty that a broken conductor will make contact
with the earthed conductor.
With the reduction in the minimum height from the ground
afforded by regulation 13, many lower voltage lines will be a,ble
to pass beneath other lines such as telegraph or telephone wires. The
Commissioners are in a position to state that in such cases the re
quirements of the PostmasterGeneral for guard wires will be satisfied
if a single earthed wire is run above the power wires for the length of
the span.
The PostmasterC4eneral has further agreed to a relaxation in the
Post Office Memorandum T.B. 80 (protection of telegraphs from
contact with low and medium pressure power circuits) by modifying
his present requirement of 4 feet clearance between low and medium
pressure power wires and telegraph lines to the extent of allowing a
minimum of 2 feet if the power line supports are placed in such a
position with, respect to Post Office wires that there shall be no
danger to men working on the Post Office poles and that there shall
be no appreciable difference in sag under the worst conditions of
ice loading.
The requirement of a bonding wire connected to the supporting
metal work of all insulators used with wooden poles appears to be
necessary in the interest of authorised undertakers ; and a suitable
earthing of metal poles is likewise necessary. These features are
therefore retained in the new Regulations.
Where lightning conductors are used with wooden poles, it is
desirable that to a height of 10 feet from the ground they should
either be insulated or connected to a continuous earth wire having
an efficient earth connection. The Commissioners have also con
cluded that where stay wires are used in conjunction with wooden
poles, it is necessary that insulators should be interposed in the stay
wire. Such insulators would not be necessary in stay wires fixed to
metal poles which are protected from leakage in accordance with
Regulation 14 (2).
REGULATION 15 (Factor of Safety of Line Conductors).
The Commissioners have made a reduction in the assumed ice
loading on higher voltage lines from onehalf of an inch to three
eighths of an inch, but, as previously indicated, have retained the
provisions of the prior Code as to wind pressure and factor of
safety. The relaxation affords a reasonable measure of relief from
the original requirement.
OVERHEAD POWER LINES
REGULATION 16 (Minimum Height of Conductors}.
In the case of higher voltage line conductors, the Commissioners
have decided to retain the provisions of the prior Code as to minimum,
height above ground. The Regulation has been amplified, however,
to deal with the minimum height of associated earth wires or auxiliary
conductors.
General Observations.
The combined effect of the preceding relaxations in the case of
lower voltage lines will be found to be such as to enable supports
of reduced diameter as well as of reduced length to be employed.
For example, making due allowance for the reduction in the height
of a lower voltage line above the ground from 20 feet to 17 feet
(Regulation 13), a wooden pole having a diameter of only 7J inches
at a distance of 5 feet from the butt will hereafter be needed where
previously a diameter of 8J inches would have been required under
the prior Code.
As a further means of facilitating the extension of overhead
distribution in rural areas, the Commissioners are prepared, under
their Regulations for securing the safety of the public and for en
suring a proper and sufficient supply of electrical energy, to give
consent in special cases to a voltage variation within the limits of
plus 4 and minus 8 per cent, of the declared voltage on rural lines for
a provisional period pending the completion of the distribution
system and its full measure of interconnection. After such pro
visional period, the variation can reasonably be limited to the normal
requirement of plus or minus 4 per cent.
Procedure.
The Commissioners suggest that authorised undertakers should
take a reasonably long view of the probable development of low
voltage overhead distribution in their area and should, where
practicable, make application to the Minister of Transport for a
consent to more comprehensive proposals than hitherto instead of
making repeated applications for individual lines, thus saving time
and expense.
With the view of expediting the consideration of applications,
the procedure formerly adopted by the Board of Trade and con
tinued by the Commissioners on behalf of the Minister of Transport,
of referring all overhead line applications to the PostmasterGeneral
for his observations will, by agreement between the Departments,
"be abandoned as from the issue of the Revised Regulations. The
APPENDIX I 213
Commissioners suggest, however, that in all cases where it is in
tended to run high voltage or extra high voltage overhead lines for
an appreciable distance parallel with Post Office overhead routes,
the authorised undertakers concerned should adopt the general
practice of conferring with the Post Office engineers in their district
prior to applying for consent to such overhead lines, so that the
possibility of inductive interference may be considered before the
route is actually fixed. It will, of course, still be necessary for the
undertakers to give statutory notice to the PostmasterGeneral in
compliance with Section 14 of the Schedule to the Electric Lighting
(Clauses) Act, 1899.
With the concurrence of the Minister of Transport, the existing
Memorandum of particulars required in connection with proposals
to erect overhead lines (Form El. C. 34) has been modified with the
view of simplifying procedure. The main alterations in the Revised
Memorandum (El. C. 53B) are as follows :
(a) Legible tracings or prints taken from 6inch Ordnance
maps may now be submitted in lieu of the maps themselves ;
and in cases relating to overhead lines at extra high voltage,
duplicate maps will not in future be required.
(b) Where an undertaker has submitted full technical details
with one application and consent is subsequently sought to the
construction of further overhead lines to the same specification,
it will not be necessary to resubmit full technical details, but
only to furnish certain limited particulars.
(c) A form of communication which the undertakers should
send to the Local Authority or County Council when proposing
to apply to the Minister for consent to erect overhead lines has
been drawn up and is included in the Revised Memorandum.
Electricity Commissionj
Savoy Court,
Strand, W.C. 2, April, 1928.
214
APPENDIX II.
ELECTRICITY (SUPPLY) ACTS, 1882 TO 1926. [El. C. 53e.]
OVERHEAD LINES.
MEMOEANDUM setting forth the information to be submitted in connection
with applications by Authorised Undertakers for the consent of the
Minister of Transport to the placing of electric lines above ground.
1. An application, for the consent of the Minister of Transport
to the placing of an electric line above ground should be formally
addressed to The Secretary, Ministry of Transport, Whitehall
Gardens, S.W. 1, but may be delivered direct to the Office of the
Electricity Commissioners who advise the Minister in connection
with the technical aspects of all overhead line proposals.
2. The application must be accompanied by the technical and
other particulars set out in the Schedule appended to this Memoran
dum duly signed on behalf of the undertakers.
3. Where the electric lines forming the subject of any application
are to be constructed in accordance with details already submitted
with a prior application to which the consent of the Minister has
been given, it will only be necessary for the undertakers to submit
certain details as indicated in the Schedule and to complete and sign
the Certificate at the end of the Schedule.
4. The undertakers should serve a notice of their application as
nearly as may be in the form set out in the next page of this Memo
randum, together with a description of the nature and position of
the proposed lines, on the local authorities in whose districts the
lines are to be placed and on the County Council in cases where a
county bridge or a main road vested in such Council is concerned.
The local authorities in England and Wales are Borough Councils,
Urban District Councils and Rural District Councils. The local
authorities in Scotland are Police Commissioners, Gas Commis
sioners, Town Councils and County Councils.
5. In making application to the Minister, the undertakers should
give the date of the service of the abovementioned notice, and a list
of the authorities upon whom the notice has been served. Where
APPENDIX II 215
possible^ the undertakers should submit evidence showing whether
or not the authorities desire to be heard by the Minister.
6. Where it is proposed to place an electric line across any land
(other than a street or public bridge) or across or along any railway,
canal, inland navigation, dock or harbour, the undertakers should
state whether wayleaves have been agreed with the owner and oc
cupier of the land or with the owners of the railway, canal, inland
navigation, dock or harbour as the case may be.
7. Attention is drawn to the revised Code of Overhead Line
Regulations of the Electricity Commissioners (Form EL C. 53) and
to the Explanatory Memorandum (El. 0. 53A) issued in connection
therewith ; and to the provisions relating to tlie approval of plans
and works contained in Section 14 of the Schedule to the Electric
Lighting (Clauses) Act, 1899, or corresponding provision in the
Undertakers' Act or Order. *
Ministry of Transport,
6 Whitehall Gardens,
S.W.I, ^7, 1928.
Form of Notice.
ELECTEICITY (SUPPLY) ACTS, 1882 TO 1926.
(Title of Order or Special Act.)
Sir,
I beg to inform you that the (insert name of applicants) have
made an application to the Minister of Transport for consent to the
placing of electric lines above ground for the purposes of the above
mentioned Order (or Special Act), and in this connection desire to
draw the attention of your Council to the provisions of Section 21
of the Electricity (Supply) Act, 1919, as amended by Section 50 of
and the Sixth Schedule to the Electricity (Supply) Act, 1926.
The Acts in question provide that where the consent of the
Minister of Transport is obtained to the placing of any electric line
above ground in any case, the consent of the local authority (in
cluding a County Council) shall not be required, anything in the
Electric Lighting Acts or in any Order or Special Act relating to the
undertaking to the contrary notwithstanding, but the Minister ^ of
Transport before giving his consent shall give the local authority
and (where it is proposed to place the line along or across any county
bridge or any main road vested in a County Council) the County
Council an opportunity of being heard.
A description of the nature and position of the proposed lines so
far as they affect your Council is enclosed herewith and I shall be
216
OVERHEAD POWER LINES
glad to learn at your earliest convenience whether (insert name of
applicants) may notify the Minister that your Council do not desire
to be heard in connection with the application.
(To be signed on behalf of the Applicants.)
NOTE. " ; "
regarc
tikis Schedule.
Schedule of Particulars.
;    ' * items (l)to(l}(d}are required in every case. With
. attention is drawn to the Certificate at the end of
(1) An Ordnance map on a scale of six inches to the mile (or a tracing or print
therefrom giving appropriate reference to the Ordnance map) must be
supplied showing :
(a) The proposed route of the line with positions of the terminal, intermediate
and angle supports, and of the "earth "plates. Any underground, portion
of the line should be shown in distinctive colour.
(b) Any existing overhead lines, whether for power, lighting, traction, telegraph
or telephone purposes, in the immediate vicinity of the proposed trans
mission lines.
(2) Working voltage
Volts.
(3) Is supply by direct or alternating cur
rent ?
(4) If by alternating current, state number
of phases and frequency .
Phase.
Cycles.
(5) Maximum amount of energy which the
line is designed to transmit, in kilo
watts ......
(6) Total route length of overhead line, in
yards ......
(7) Conductors :
(a) Number .....
(b) Material used ....
(c) Solid or Stranded ....
(d) Sectional area of each conductor and
when stranded, the number and
diameters of wires
*(o) Height of lowest conductor (or earth
or neutral wire if forming part of the
conducting system) above ground at
pole, in feet ....
*(f ) Sag of lowest conductor (or earth or
neutral wire if forming part of the
conducting system) at temperature
of 122 Fahrenheit on maximum
span, in feet ....
*(g) Minimum height above ground of
lowest conductor or wire between
poles on maximum span, in feet .
*(h) Breaking load of materials in tons
per square inch ....
*(i) Elongation of conductor in length of
10 inches on breaking, per cent. .
(a),
(b),
(c)..
(d).,
(e).
(f).
(g)
(h)..
APPENDIX II
*(8) Earth Wire (not forming part of the con
ducting system) or auxiliary con
ductor :
(a) Size ......
(a)
(b) Description .....
(b)
(c) Height above ground at pole, in feet
(c)
*(9) Span between poles or other supports :
(a) Average span, in feet
(a)
(b) Maximum span, in feet
(b)
(10) Poles :
(a) Class of pole to be used, i.e. Wood,
Steel Tubular, Lattice or other ma
terial. See paragraph (13) below
(a)
(b) Diameter of pole at top, in inches
b)
(c) Diameter of polo at 5 feet from butt
in inches ....
(c)
(d) Depth of pole in ground, in feet
(d)
(e) Overall length of pole, in feet
(e)
(f ) If wooden polos are used, the nature
of the timber ....
(f)
(g) If steel tubular poles are used, the
thickness of metal, in inches
(g)
(h) Breaking stress of steel used, in poles,
in tons per square inch
(h)
*(11) Type of automatic protective device .
*(12) Earth plates :
(a) Tvpe
(a)
(b) Dimensions .....
fb)
(c) Metal .....
(c)
(d) Number proposed
(d)
*(13) A drawing (scale to be stated and to be not
loss than onehalf inch to the foot) of each type
of pole proposed to be used, with dimensions as
in Fig., must bo supplied showing :
(a) Details of stays and struts.
(b) Crossarms.
(c) Insulators.
(d) Arrangement of conductors and their sizes
indicated against each insulator,
(o) Safety arrangements at road, railway and
canal crossings.
(f) Earth wire and earth plates.
Tf steel lattice masts, reinforced concrete poles or
supports of special design are proposed to be used,
stress diagrams with detailed calculations must be
submitted in addition to the drawing referred to
above.
The particulars set out against items of
the above Schedule relate to tho overhead lines
forming the subject of the application made on
by the
(Signed
Electrical Engineer to the Undertakers.
Diameter
I
'//y////
T
,T
Diameter
218 OVERHEAD POWER LINES
* Certificate.
(In cases where details (7) (e) to (13) submitted with a prior applica
tion are applicable.)
I HEREBY CERTIFY that the overhead lines forming the
subject of this application will, so far as items (7) (e) to (13) of the pre
ceding Schedule are concerned, be constructed in accordance with
the details submitted in connection with a prior application made
on to which the consent of the Minister of Transport
was given on
(Signed)
Electrical Engineer to the Undertakers.
219
APPENDIX III.
POST OFFICE ENGINEERING DEPARTMENT.
[E. in C. 231.]
MEMORANDUM on Protection of Overhead Telegraph or Telephone
wires at Crossings of High or Extra High Pressure overhead
Power Linen.
THE Electricity Commissioners' Regulations for Overhead Power
Lines stipulate that where a line conductor crosses over or under or
is in proximity to any other overhead wire, precautions shall be
taken by the undertakers to prevent contact, due to breakage or
otherwise, between the line conductor and the other overhead wire,
or between the other wire and the line conductor.
"Where the power circuit is classified as high or extra high pressure
it is the practice to arrange for either the power wires or the tele
graph and telephone wires to be placed underground at crossings.
Where the Post Office circuits are local and there is little loss in
efficiency by placing them underground this method of protection
will be adopted (otherwise the power line should be placed under
ground for the requisite distance).
In cases where there are serious objections to either the telegraph
or the power line being placed underground the method shown in
the annexed sketch will be adopted, the protection taking the form
of a substantial cradle guard. The power line and the guard should
comply with the following conditions :
(1) The routes must cross at right angles and continue at right
angles for a distance of not less than 20 yards on each side of
the crossing. In difficult cases, however, a deviation up to 30
degrees from the right angle will be accepted for a straight
through crossing.
(2) The power lines must cross above the telegraphs or tele
phones.
(3) Duplicate conductors lashed every 5 feet and terminated
on separate insulators must be provided for each power wire at
each end of the crossing span ; alternatively a single conductor
will be accepted, provided that it is stranded and used with
duplicate insulators, bridles, and the earthing device specified
by the Electricity Commissioners :
(4) The poles oF structures supporting the power wires at the
crossing span must be sufficiently strong to serve as terminals
should the wires in adjacent spans break.
220
OVERHEAD POWER LINES
Q:
APPENDIX III
The deflection of the structures under such, conditions must
be less than would permit the power conductors to sag on the
guard.
The poles should preferably be of steel or iron but if of wood
all metal fittings shall be connected to earth by low resistance
conductors as specified for the guard under condition (5).
(5) The independent poles supporting the cradle guard may
be of wood, iron, steel or reinforced concrete. The clearance
between guard and power conductors must be sufficient to pre
vent contact under the worst possible conditions, otherwise than
by breakages of conductors.
The top or outside wire on each side of the independent cradle
guard shall be so arranged that lines drawn upwards from them
towards the centre at an angle of 45 degrees will totally enclose
the power wires together with any telephone control or other
wire belonging to the power system.
The guard to be made of wire of not less than 7/14 S.W.G.
galvanised steel or hard drawn copper.
The cradle will be crosslaced every two feet above the tele
graph wires to a distance of 6 feet beyond the wires on each side.
The guard to be connected to earth at each end of the crossing.
The resistance of guard to earth shall not exceed 1 ohm or 60/A
ohms, whichever is the smaller value. (A = max. current of
system.)
The earth and earth connections must be capable of carrying
the maximum current which can flow to earth in the event of a
contact between a power conductor and the guard.
In the case of wood poles, the earth connection shall be so
grooved into the pole that there will be no danger of the wire
being tampered with.
(6) The clearance between the guard and the telegraphs must
not be less than 3 feet.
(7) The poles, structures and wires of the power lines must
be constructed with the factors of safety laid down in the Elec
tricity Commissioners' Regulations for Overhead Lines.
(8) The structure supporting the power line crossing span to
be placed in positions free from, risk of damage by traffic.
(9) Alternatively to the provision of an independent guard,
the erection of a cradle guard on the power circuit supports will
be accepted, providing the structures are of steel or iron, very
stable in design and that other conditions are satisfactory.
(10) Detailed drawings and plan to be submitted to Bngineer
inChief, Post Office, for approval, the erection and maintenance
to be to the satisfaction of the EngineerinChief.
APPENDIX IV.
POST OFFICE ENGINEERING DEPARTMENT. [T.E. 80.]
MEMORANDUM: on Protection of Telegraphs from Contact with Low and
Medium Pressure Power Circuits (excluding Traction Circuits).
1. The Electricity Commissioners' Regulations for Overhead
Power Lines stipulate that where a line conductor crosses over or
under or is in proximity to any other overhead wire, precautions
shall be taken by the undertakers to prevent contact, due to breakage
or otherwise, between the line conductor and the other overhead,
wire, or between the other wire and the line conductor.
2. For the purposes of this Memorandum the expression " tele
graphs " includes all telegraph and telephone conductors and also
stay wires; and the expression "power circuit" means any con
tinuous current * or alternating current * power circuit (other than
a traction circuit) so arranged that the maximum pressure between
any two conductors of a circuit entirely insulated from earth or
between any conductor and earth, in the case of a circuit earthed at
the power station, substation or transformer, does not exceed 650
volts.
3. When the pressure to earth, or between any two conductors
of an unearthed system, does not exceed 60 volts A.C. or 120 volts
C.C., no guarding is required.
4. When the pressure to earth, or between any two conductors
of an unearthed system, exceeds 60 volts A.C. or 120 volts C.C.,
guarding is required :
(a) at each point of crossing or overhanging ;
(6) in the case of parallel lines, where the vertical distance
between any telegraph, and any power wire exceeds
the horizontal distance  (see Pig. 1),
and the approved arrangement and the scope of each are as follows :
* In further references to " continuous current " and " alternating current "
the abbreviations " C.C." and " A.C." respectively are used.
f This corresponds to the 45 rule applied to electric tramway and trackless
trolley systems.
APPENDIX IV
SCOPE.
Maximum Pressure
to Earth or between
Conductors of Un
earthed Systems,
System. volts.
I. The disposition of permanently A.C. 250
earthed power conductors * so that C.C. 650
they act also as guard wires.
II. The use, for potential conductors, of A.C. 250
wires insulated with an approved C.C. 650
weatherproof covering f (bare wires
being permitted for permanently
earthed conductors).
III. The use of any form of covered power A.C. or C.C. 650
conductors (including leadcovered
cables) supported by earthed bare
suspending wires (including neutral
conductors).
IY. The use by the Post Office of insulated A.C. 250
conductors for the telegraphs, other C.C. 650
than the telephone trunk circuits.
V. The provision of independent guard A.C. or C.C. 650
wires.
5. The details of the normal requirements, which in some cases
depend on the design of the power circuit and its position relatively
to the telegraphs, are set forth in the following pages ; but, occa
sionally, e.g. in very exposed situations, it is necessary to impose
more stringent requirements.
6. In all cases, i.e. irrespective of the pressure of the power circuit,
a clearance of 4 feet should normally be given, but where it would be "
difficult or costly to provide more than 3 feet this will be agreed.
Further, at crossings a clearance of 2 feet will be agreed, at the
discretion of the Post Office Sectional Engineer, in cases where the
power line supports are placed in such a position with respect to
Post Office wires where there will be no appreciable difference in
sag with changes of temperature or under the worst conditions of
ice loading, and that there is no danger to men working on the P.O.
poles. If the P.O. line is not fully developed additional clearance
* The term " earthed (power) conductor " as used in this Memorandum means
" permanently earthed " and includes " neutral conductor."
t Other braided conductors, with or without rubber, fall under arrangements
I., III. and V.
OVERHEAD POWER LINES
may be necessary at the outset to provide for the ultimate con
ditions.
7. Telegraphs endangered by a power circuit are equipped with
internal protective devices, i.e. fuses and heat coils, and the erection
of a power circuit may involve the provision of such devices.
8. When the necessity for protection arises from the erection of
telegraphs, i.e. the telegraphs are " second comer," the Postmaster
General as a concession will bear the cost of any fuses and heat coils
installed and also the cost of the most economical method of guarding
having regard to the ultimate conditions ; but in all other cases the
whole cost of protection falls upon the undertaker.
Power
Power
T f 1 T
1 i f 7
f M 1
f f t t
t t t i
t.. t M
i
i
h
GUARD WHEN "a" is LESS THAN "b".
FIG. 1.
(I) Permanently Earthed Conductors arranged to act
as Guard Wires.
9. The earthed conductor shall consist of copper wire, not lighter
than No. 11 S.W.Gr., which shall be earthed at one point only, that
is, at the generating station, substation, or transformer. The
earth connection must be permanent and the electrical continuity
of the wire must be maintained at all times.
(A) TELEGRAPHS ABOVE A POWER CIRCUIT.
(i} Crossings at Angles greater than 30.
10. An earthed conductor shall be erected above the potential
conductors Figs. 2, 3 and 4.
11. "Where a vertical formation is used for the power wires as in
APPENDIX IV
Fig. 2 an earthed conductor shall be erected above the potential
conductors in the same vertical plane with a minimum clearance of
8 inches to the highest potential wire.
12. Where the power wires are erected with any other than the
vertical formation an earthed conductor shall be erected uppermost
in such a position that lines drawn from it to the outermost potential
wires will not make a greater angle than 45 with the vertical. See
Fis. 3 and 4.
Co/jc/uc&or* W/re
Potent /a/ Conductor W/re.
Pro. 3.
PIG. 2.
FIG. 4.
('ii) Crossings at Angles less than 30 <m$ Parallel Lines.
13. Where the angle of crossing is less than 30 two earthed
conductor wires shall be provided if in the opinion of the Post Office
Sectional Engineer the conditions as regards danger are not satis
factorily met by the provision of one earth guard wire. Figs. 5, 6,
7 and 8.
Crosslacing may also be required if there is direct overhanging,
and "this should be provided by means of wire placed at intervals of
15
226 OVERHEAD POWER LINES
not more than 6 feet for such a distance as may be stipulated by the
Post Office Sectional Engineer.
(B) TELEGRAPHS BELOW A POWER CIRCUIT.
(i) Crossings at, or approximately at, Right Angles.
14. The earthed conductors shall be erected as shown in Fig. 9,
which indicates three variations in their position relatively to the
potential conductors.
Conductor W/re .
Potent /a/ Conductor fY/re
>^ ^, 
M/n.8' M/n.8" Min.8"\
\M P . ^.;
FIG. C.
\Afin. I
\8" i
y i
FIG. 7.
FIG. S.
15. (1) When the earthed conductors are erected below any
"potential conductor, including a switch wire, the overlap a shall be
greater than the vertical distance 6 between the highest potential
conductor and the plane of the earthed conductors.
(2) In all cases the earthed conductors shall be connected by
cross wires of the same gauge passing under the potential conductors,
the cross wires being spaced at intervals of not more than 6 feet for
a distance of 18 feet on each side of the crossing, except where there
is a pole within 18 feet, in which event the cross wires need not extend
beyond the pole.
APPENDIX IV
227
(ii) Diagonal Crossings and Overhangings.
16. Fig. 9 applies, "but tlie earthed conductors will be crosslaced
at intervals of 6 feet, for such a distance as may be stipulated by the
Post Office Sectional Engineer.
(Hi] Parallel Lines.
17. Fig. 9 applies, but the earthed conductors will be crosslaced
at intervals of 6 feet throughout the section affected, i.e. falling
within the scope of paragraph 4 (b).
A.
A
,'*
cross /actn
n J
cross lacino
a to bt greater llum b
r r i
^ Telegraphs bghuj
^^
eufral
A 4 4 k
46/1 >
t,
cross lac/na
eutral
FIG. 9.
(II) Power Conductors Insulated with Approved
Weatherproof Covering.
18. Conductors covered with a satisfactory weatherproof in
sulating material similar to that used by the Post Office at power
crossings see (IV) below will be accepted as affording adequate
protection, provided that the pressure to earth of the power circuit
does not exceed 250 volts A.C. or 650 volts C.C. Further, within
these limits of pressure, bare wire may be used for permanently
earthed conductors.
228 OVERHEAD POWER LINES
The covered conductor must withstand specified electrical tests
of a searching character.
These tests, together with the significant clauses in the Post Office
specification for the type of covered conductor referred to are given
in Appendix I. Conductors covered with this type of insulation
can be obtained from the leading British makers of insulated wire
and, if purchased to the specification from manufacturers approved
by the Post Office, will be accepted as satisfactory, subject to the
proviso that the PostmasterGeneral reserves the right in any par
ticular case to ascertain by tests on samples of the wire whether the
specification is being complied with. Should the tests on the samples
show that the wire is not to standard, approval of its use will be
withheld.
Varnished cambric will be accepted as an alternative to paper
in the make up of the insulating covering.
(Ill) Covered Power Conductors Supported by Earthed
Bare Suspending Wires.
19. Any covered power conductor will be accepted as satis
factorily guarded if it is suspended from an efficiently earthed bare
wire, by means of uninsulated ties. The distance between such ties
shall not exceed 2 feet, and the suspending wire shall be earthed at
both ends and, if necessary, at intermediate points, so that the dis
tance between any two earth connections does not exceed 200 yards.
(IV) Insulated Conductors for the Telegraphs.
20. "When the pressure of the power circuit does not exceed 250
volts A.C. or 650 volts C.C., the Postmasterden eral is prepared to
erect or substitute insulated wire, instead of bare wire, for the tele
graphs, provided that a small number of telegraph wires is concerned,
the route is not likely to grow, and that the efficiency of the circuits
will not be impaired.
Generally, the arrangement is not applicable to Post Office lines
carrying telephone trunk circuits.
(V) Independent Guard Wires.
21 . Guard wires should be, in general, of galvanised steel, mini
mam gauge No. 8 S/W.G. or 7/16 S.W.G., but in manufacturing dis
tricts where such wire is liable to rapid corrosion, bronze or hard
drawn copper wires of equivalent strength should be used.
The supports for the guard wires should be rigid and of sufficient
strength for their purpose, and at each support each guard wire
should be securely bound in or terminated.
APPENDIX IV 229
Each guard wire, including all the longitudinal wires forming a
cradle guard, must be well earthed at both ends, and at intervals
.of not more than 200 yards.
Each Earth should be made by means of a permanent connection
to a water main, or by means of a substantial castiron earth plate
fitted with a strand of copper wire of sufficient length to admit of
the joint being made above ground. When first erected the resist
Earthed Guard W/re.
@ Potential Wire.
t
! %'
8 XL
\
\
FIG. 10.
/TV
/>
Mm.
FIG,
ance to earth of the guard wires should be tested, and periodical
tests should be made by the undertaker to prove that each earth
connection is efficient.
(A) TELEGRAPHS ABOVE A POWER CIECTJIT.
(?) Crossings at Angles greater than 30.
22 Where a vertical formation is used for the power wires, as in
Fig. 10, a guard shall be erected above the power wires m the same
230
OVERHEAD POWER LINES
vertical plane with, a minimum clearance of 8 inches to the highest
power wire.
23. Where the power wires are erected with any other than a
vertical formation a guard wire shall be erected uppermost in such
a position that lines drawn from it to the outermost potential wires
will not make greater angles than 45 with the vertical plane. See
Figs. 11 and 12.
>_ >
M/r>.8* M/n.8"
8
Via. 13.
FIG. 14.
* >i
/ X >// lt>
M/n.o P
' l
\Afrn.
M/n.8"
Af/n.8"
M/n.
FIG. 15.
FIG. 16.
(ii) Crossings at Angles less than 30 and Overhangings.
24. Where the angle of crossing is less than 30 two earthed guard
wires shall be provided if in the opinion of the Post Office Sectional
Engineer the conditions as regards danger are not satisfactorily met
by the provision of one earthed guard wire. Figs. 13, 14, 15 and 16.
Crosslacing may also he required if there is direct overhanging,
and this should be provided by means of wire placed at intervals of
not more than 6 feet for such a distance as may be stipulated by the
Post Office Sectional Engineer.
(B) TELEGRAPHS BELOW A POWER CIRCUIT.
(i) Crossings at, or approximately at, Right Angles.
25. An earthed cradle guard (Fig. 17) shall be erected between the
power circuit supports.
APPENDIX IV
(u) Diagonal Crossings and Overhanging s.
26. Fig. 17 applies, but the cradle guard will be provided for
such, a distance as may be stipulated by the Post Office Sectional
Engineer.
(m) Parallel Lines.
27. Fig. 17 applies, but the cradle guard will be provided through
out the section affected, i.e. falling within the scope of paragraph
4(6).
mm 4 feet
mesh not to exceed
2 feet x S feet
X x x x .erTeleqrapbs
xxx x J '
Fro. 17.
APPENDIX I.
SIGNIFICANT CLAUSES 01? THE P.O. SPECIFICATION FOR CONDUCTORS
INSULATED WITH APPROVED WEATHERPROOF COVERING.
GENERAL.
The completed wire shall consist of a hard drawn copper wire insulated with two
layers of impregnated paper covered with a layer of cotton lappings and o ottou
braiding impregnated with a weatherproof composition.
' DIELECTRIC! AND WEATHERPROOF COVERING.
Thetwc'. . ' "!!"" . ' . "i 1 * v.y '' .'i 'i j. .i .: hi opposite
directions ' : ." '!'... .y ' i r i. .: . : .. , oroximately
5 turns in 3 inches. The paper shall be manilla of approximately 280 inch wide
and 006 inch thick, treated with linseed oil.
The cotton lappings and braiding shall be thoroughly impregnated with a mixture
composed of approximately :
Red Lead . . . . .72 parts by weight.
Linseed Oil ](>
Paraffin Wax , . . . ]2
The paraffin wax before being used shall be rendered anhydrous by being heated
to a temperature of 300 to 350 F. until all water is expelled.
The completed wire shall be finally passed through a bath of anhydrous paraffin
wax at a temperature of 150 to 200 F,, so that the covering is left with a fairly
smooth and glossy surface.
32 OVERHEAD POWER LINES
INSULATION TESTS.
The completed wire shall pass the following tests not less than 14 days after
anufacture :
(a) A piece of tinfoil 6 inches in length will be lapped closely round the wire
at any points selected by the Inspecting Officer. An insulation test made
between the conductor and the tinfoil, using 1 000 volts for the test, shall give
a resistance of not less than 100 megolims.
(b) A similar test to (a), made after the coil has been immersed in water for
24 hours, shall give a minimum resistance of 2 megohms.
(c) Insulation tests will be made between bare wire closely lapped round
the exterior of the completed wire and similar bare wire lappings inches
distant. Three laps of the bare wire will be made at each point. The insula
tion shall be not less than 10 000 megohms when tested with I 000 volts.
(a!) A similar test to (c), made after the coil has been immersed in wa.ter for
24 hours, shall give a minimum resistance of 100 megohms.
32,95
GENERAL INDEX.
'A'
POLES, calculations for founda
tions, 120.
scarf joint, 122.
foil nd ations, ordinary type, 127.
Anchora type, 131.
Rutter type, 131.
method of construction, 120.
strength of, 113.
Aluminium conductors, 185.
steel conductors, 188.
Anchorages, 17(5.
Anchora typo of " A " polo foundations,
131.
Angle of reposo of soil, 179.
swing of conductors, 42, IOC.
Angles in lino, 58, 103.
properties of steel, 71.
Arms, cross, oak, channel and angle, 70.
Armstrong, Addison & Co., viii.
Arrangement of conductors on polo, 44.
Association, British Engineering Stand
ards, vii.
AKELISED wood, 70.
Basalt, fuzed, 09.
Base plates, angle polos and struts, 108,
172.
iron poles, 139.
Basic loading conditions, conductors, 10.
poles, 05,
Bending moment on pole, 95.
Binding in, 64.
wire, approximate quantities required,
60.
Bird guards, 41 .
Blocks, brace, 127.
foundation, 111, 110.
kicking, 128.
scarf, 123.
stay, 170.
Bolts, particulars of, 75.
Bonding, 103, 194.
Brace, blocks, 127.
Brackets, insulator, 458, 87, 91.
British Engineering Standards Associa
tion, vii.
Bronze conductors, 1R3.
Buckling strength of poles and struts,
118, 107, 172.
c.
CADMIUM copper conductors, 185.
Oallender's standard polo fittings, 48.
Cap and pin tensioning insulators, 08.
Catenary curve, 18.
Cementing insulator on to pin, 01.
Channel iron cross arms, 78.
polos, 140.
Chart for determining economical span,
.101.
Choice of working voltage, 2.
Clearance between conductors, 40.
Clips, conductor, for pin insulators, 03.
Coach screws, 70.
Cobra impregnnting solution, 04.
Coefficients of linear expansion, 184,
self induction, 2.
Cohesion of soil, 1.70.
Comparison between copper and alumi
nium, 180, 187.
other materials, 184.
stool, 191.
Compound channel iron poles, 144.
wood poles, 115.
Concrete, allowable stress, 144, 154.
composition, 144, 154.
foundation 1 ?, 143.
poles, 154.
Conductors, aluminium, 185,
steelcored, 1F8.
arrangement of, on polo, 4.4,
bronze, 183.
'.: . 185".
copper, sag and stress calculations, 18.
particulars of, 4,
critical temperature, 20.
erection sags, H.V. lines, copper, 24.
L.V. lines, copper, 20.
tensions, H.V. lines, copper, 25.
L.V. lines, copper, 27.
sags and tensions, steel, 190.
solid t. stranded, 37.
spacing, 42,
234
OVERHEAD POWER LINES
Conductors, steel, 190.
Construction of " A " pole. 120.
Costs of supports, 103.
Creosote i 1 ' *'. Oi.
Critical i . ' .
Cross arms, 70.
Crossings, railways, 197.
roads and canals, 197.
telegraph and telephone wires, 198.
Cubic equations, 23.
Curves. See Table XL
D,
'ANGER notices, 207.
Deflection of single wood poles, ] 13.
steel tubular poles, 140.
Depth of pole foundations, 109.
Device, earthing, 196, 197, 205, 207.
Dimensions of standard poles, 97.
Displacement of conductors in wind, 35.
Distribution, high voltage, 1.
low voltage, 16.
Double insulators, 49, 196, 197.
Dry spark over voltages, 56.
Duplicate insulators, 49, 196, 197.
Dynamometer, 38.
IARTHED neutral conductor, 194.
Earthing device, 19C, 197.
Earbhs, particulars of various, 179.
Earth wires, 193.
Ebonestos insulators, 68.
Eccentric loading, 72.
Economical span length, 100.
Economy due to steel conductors, 190.
Elastic extension and contraction of con
ductors, 20 ei seq.
Elasticity, moduli of, 186.
Electricity Commissioners' Regulations,
201.
Energy loss in conductors, 9.
Erection sags, copper conductors, 24, 26.
and tensions, steel conductors,
190.
tensions, copper conductors, 25, 27.
Euler's formulae for struts, 74, 118, 167.
Expansion, coefficients of linear, 186.
JD ACTORS of safety, 203, 204.
Ferro concrete poles, 154.
Fir poles, 94.
Fittings, pole, calculations, 70.
types of, 44.
Flash over voltage, insulators, 56.
Flexibility of poles, 114.
Foundations, " A " poles, 126.
Anchora type, 131.
Rutier type, 131.
Foundations, iron poles, 13S, 143, .149.
Rutter poles, 116.
single wood pole.i, 105.
Frequency of line vibrations, 43.
supply, 3.
Fuzed basalt, 69.
G
TALVANISED stool conductors, 190.
stay wire, ] 70.
Glass insulators, 68.
Glazing of insulators, 55.
Graphic solution of cubic equations, 23.
Gripper for shackle insulator, 1(54.
stay, 104.
Guarding in general, 193.
Guards, bird, 41 .
climbing, 207.
wire, 48, 53, 196, 197.
H
__ POLES, 133, 174.
Height of conductors and wires from
ground, 40.
Hewlett tensioning insulator, 67.
High voltage lines, basic loading con
ditions, 19.
erection sags and tensions, 20.
" ' . " 'ions, .194, 195.
of conductors
in wind, 35.
loading on conductors, 20.
spacing of conductors, 42.
I,
CE loading, 20.
Impregnation of poles, 94.
Inductance, 2.
Insulated hard drawn copper con
ductors, 198.
Insulators, pin s at angles, 58.
attachment of conductor, 62.
to pin, 61.
electrical design, 55.
mechanical design, 56.
standard tests, 56.
stay, 194.
tensioning, 66.
Iron and steel conductors, 190.
poles, 135.
Ironwork, pole, 44, 70.
JL\ ALANITE insulators, 68.
Kicking blocks, 128.
L:
. INK type tensioning insulators, 67.
Losses, energy in conductors, 9.
Low voltage lines, basic loading condi
tions, 19.
GENERAL INDEX
Low voltage lines, erection sags an
tensions, 20.
safety precautions, 193, ] 94.
AKING off stay wire on pole, 171
thimble, 170.
Metal cap tension ing insulators, (18.
Moduli of elasticity, concrete, 156.
conductors, 184.
steel and wood, 73.
steel sections, 71.
Modulus of rupture, red fir, 95.
Moments of inertia of steel sections, 71.
Most economical span, 100.
ATURAL frequency of conducto
swings, 42.
Neutral conductor, L.V. and M.Y
systems, 1 94, 205.
"Norwegian red fir poles, 94.
Number plates for poles, 207.
VERTURNING moment, poles, 109
_L ARABOLA, 18.
Pern ax insulating material, 42.
Physical constants of conductor mate
rials, 184.
Pins, insulator, 56.
Pin type insulators, 56.
Plaster of Paris, 62.
Plates, base for iron poles, 139.
danger, 207.
earth, 194, 206.
number, 207.
stay, 182.
Poles, ferroconcreto, 154.
fittings, 44, 70.
foundations, 105, 120, 138.
iron, channel, 141.
tubular, 135.
ironwork, 44, 70..
wood, simple, 94.
twin, 115.
" A " tvpe, 116.
" H " type, 133.
_ Rutter type, 116.
Porcelain insulators, 55.
Post office regulations, 219.
technical instructions, vi.
Power factor, correction, 10.
effect on line losses, 9.
voltage regulation, 8.
Precautions, safety, 193.
Puncture voltage, insulators, 56.
_O ADII of gyration of steel sections
71.
Railway crossings, 1 97.
Rake, at angle poles, 169.
Reactance, 3.
Regulation of sag of conductors, 38.
voltage, 1.
Regulations, Electricity Commissioners,
201.
Post office, 219.
Reinforced concrete poles, 1 54.
Repose of soil, angles of, 179.
Road crossings, 1 90.
Rods, stay, 170,
Rupture intensity of soil, 108, 181.
P '!. " paint, 72.
" A " pole fotvndation, 181.
AFETY precautions, 103.
Sag adjustment, sighting, 88.
curves, 29, ,'50, 31.
by frequency of swings, 42.
dynamometer, 3<S.
Sagtcmpcraturc calculations, 18.
Sap wood, 70.
Scarf joint, 122.
Screws, coach, 70.
Sections, steel, particulars of, 71.
Selfinduction, 2.
Service lines, 194.
Shackles, 65, 66.
Shropshire, S. and W. Elec. Power Co.,
52.
Singlephase voltage drop, 7,
Skin effect, 2, 192.
Soil, particulars of various, .179.
Spacing of conductors, horizontal, 42.
vertical, 44.
Specifications, Britisli Standard, vii.
Standard poles, 97.
span lengths, 104.
specifications, vii.
"Stay Mocks, 176.
insulators, 68.
rods, 170.
wires, 170.
attachment to pole and thimble,
170, 171.
teatite insulators, 68.
teel conductors, 190.
cross arms, 78.
poles, channel, 140.
tubular, 136.
sections, standard, particulars of,
71.
truts, 166.
ynohronous swinging of wires, 42.
236
OVERHEAD POWER LINES
J_ ABLES. flee list, x.
Tackle, trussing for "II " poles, 174
Taper of wood poles, 98.
TeJenduron insulators, OS.
Telephone and telegraph crossings, 198,
219.
Temperature, critical, 20.
Temperaturesag calculations, IS.
Tensioning insulators, GO.
Terminal poles, 1(>9.
Terminating conductors, GO.
Threephase voltage drop, 7.
Tolerance, voltage, 1.
Transmission, energy loss in, 0, 12, 13.
voltage drop in, S, 11.
Trussing tackle for " JT " pole, 174.
Tubular steel poles, 136.
Twiss ten sinning insulator, 67.
u
LTIMATE stresses of materials, 73.
V GUARDS, 52.
Variation of reactance with spacing, 3.
Vector diagram of voltages, l>.
Vertical spacing between conductors, 44.
Voltage, automatic regulation, 1.
choice of working values, 2.
regulation, 5.
tolerances, 1.
ADE, Gabriel & English, viii.
Wayleaves, v, 103.
Weights, cement and concrete, 144, 154.
conductor materials, 184.
insulators, 57.
steel sections, 71.
various soils, 1 79.
wood poles, 97.
Wet spark over distances, insulators, 50,
57.
Wire guards, 4853, 195.
Wood cross arms, 70.
polos, compound Butter type, 110.
" A " type, 116.
"H"type, 133.
simple, 94.
standard sizes, 97.
Working stresses, bolts, 75.
soil, 181.
steel. 74.
timber, 76,
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TRUSTED IK (IRMAT BRITAIN HY THE ABERDEEN UNIVERSITY KRESS, ABERDEEN